How do you solve a problem like Connie?

When she was two years old, Connie Fisher's grandfather, a signwriter by trade, gave her a music box with her initials inlaid in gold leaf. When opened, it played "Edelweiss", the glutinous ditty sung by Captain Georg von Trapp in The Sound of Music in celebration of the (unofficial) national flower of Switzerland, as he exhorts it to "bloom and grow for ever". Given that Connie would mature, 21 years later, into Maria, the star of The Sound of Music, it was spookily prescient of her grandfather. Connie felt it herself: "It felt like things were to be."

Fisher is a firm believer in the non-random nature of events. In an interview in August, she said: "In a split-second, life can change. I was working in telesales a year ago, earning £150 a week. People talk about being in the right place at the right time, but it really is true. Every train I miss, I think: is that a good or a bad thing?"

What does Fisher think about the new turn of events in her career? Has Fate decided that things were not to be after all? Or is it another missed train, whose absence will turn out to be a boon after all? After her stint in The Sound of Music ends next Saturday (her replacement is Summer Strallen), Fisher was booked to appear for five performances of Simply Bernstein at Cadogan Hall in Sloane Square in early April. It was to have been her first stage appearance since playing Maria. But the producer, Daniel Wood, has cancelled the show "for personal reasons" and is refunding the audience's money.

Worse, her 13-venue nationwide lap of triumph, titled Connie Fisher on Tour, beginning on 31 May in Blackpool and ending in late June in Birmingham, has also been cancelled. The producers, The Really Useful Group and Phil McIntyre, cite "a clash of commitments" as the reason. The real reason seems to be a disastrous lack of interest. At the Cardiff International Arena, which can hold audiences of 4,500, only 200 tickets had been sold after a month.

You would need a heart of pure flint not to feel sorry for Fisher, whose 25th birthday falls on 17 June, a day before she was due to appear at the Liverpool Empire. Can it possibly be that the career of Britain's most recent national sweetheart has run slap-bang into a wall at just 25? What does it tell us about the ordeal of being a showbiz high flyer in the West End today? And is there a palpable difference in being a real star and a television-generated simulacrum?

Critics, inside showbusiness and on the media margins, have often raised doubts about the authenticity of Fisher's talent. She hit the jackpot by appearing on the BBC's How Do You Solve a Problem Like Maria? an unprecedented hybrid of reality television and real-life talent search which kept the nation amused through the late summer of 2006, as a line-up of shining-eyed hopefuls warbled over- familiar tunes to a panel of judges, while the god-like figure of Andrew Lloyd Webber presided over events from a throne.

Lord Lloyd-Webber, who produced the talent show, was about to revive The Sound of Music at the London Palladium, starting in November. As the TV show began in August, he had still to cast the central role, Maria. Could he find her in time? Would she have what it takes to succeed in the most competitive showbiz climate in the world, after Broadway?

Ten women competed for the role, singing one number every week for the voting television audience. On 12 August, Fisher sang Aretha Franklin's "You Make Me Feel Like a Natural Woman", and the nation's watchers sat upright. Next week she nailed Lulu's "Shout" with the energy of a 17-year-old. The nation rubbed its eyes. In succeeding weeks she tore into "My Baby Just Cares For Me", "Diamonds Are a Girl's Best Friend" and "If I Can't Have You". The nation held its breath, bit its lip and crossed its fingers. In the grand final on 16 September, she and Helena Blackman duelled it out; 1.5 million votes were cast; and the ingénue with the Maureen Lipman looks was on her way to the Palladium. "I won't let you down," she told fans. "Every night will be an opening night. Thanks for making my dreams come true."

Small and white, clean and bright, Connie Fisher was never going to be a glamour-puss or a leading dramatic actress. She was wholesome, tomboyish, enthusiastic, toothily keen to please – the perfect Maria, in other words, however hard it might be to cast her in worldlier roles. But neither was she the girl-from-nowhere that audiences assumed her to be.

She was born in Lisburn, Northern Ireland, in 1983. Her father was in the Army Signals Corps, her mother the PA of a car company CEO. The Fishers moved to Blandford, Dorset, where they lived until Connie was six, then to a village near Haverfordwest in west Wales. It was a happy childhood, spent mostly on a farm. She learnt to play the flute, speak Welsh ("My roots will always be Wales," she says) andlooked after a guinea pig called Mr Ruffles.

Her parents separated when she was 11 and she threw herself into singing and performing. Her grandmother encouraged her to compete in local eisteddfods, which she did, winning several talent shows in 2002. She joined the local opera society and performed in The Pirates of Penzance and South Pacific.

With the National Youth Music Theatre she toured Japan in a musical called Pendragon, playing the lead, Morgan le Fay. She won a scholarship to the Mountview Academy of Theatre Arts and emerged in 2005 with a first-class degree in musical theatre.

There followed a year of intense frustration, in which she kept narrowly missing the big time. She auditioned for parts in shows such as The Woman in White, Evita and Phantom of the Opera. and found herself in the final two or three hopefuls for Mamma Mia, Bad Girls and Chitty Chitty Bang Bang. While enduring these always-the-bridesmaid excursions, she worked in media telesales and wiped tables for a spell as a waitress in Pizza Express.

By the time she took the stage in How Do You Solve... a determined amateur chanteuse and hoofer lurked behind Fisher's shining brown eyes. A lot of tantalising not-quites and nearly-theres had brought her close to giving up and accepting that Fate had other plans for her. But she never gave up.

The Sound of Music premiered on 15 November 2006, to exceptional reviews. "Connie makes an absolutely enchanting Maria in Jeremy Sams's opulent production," wrote Charles Spencer in The Daily Telegraph.

"Connie is enchantingly fresh and ardent, and she sings in a voice that can range from purity to soft tenderness," cooed Paul Taylor in The Independent.

"She has not only an excellent voice box," observed Benedict Nightingale in The Times, "but a quirky charm and a sense of vulnerability not matched by Julie Andrews in the movie."

Could any actress/singer wish for more? If so, more was coming. In February last year, Fisher was named Most Promising Newcomer in the Theatregoer's Choice Awards – but trouble was also coming. After 98 performances on the trot, Fisher began to suffer. The strain of doing eight shows a week was starting to tell (it probably wasn't helped by the tabloids' prurient interest in her alleged dalliance with a sound engineer).

By February, the newspapers reported that she had missed nearly a quarter of her performances, was suffering from exhaustion and might have to quit the show. "She's excited and petrified," a friend helpfully explained, "but she has perhaps bitten off more than she can chew." "She's been thrust into this spotlight," complained the actress Claire Sweeney, ignoring the evidence that Fisher had been seeking the spotlight for years.

In early March, the producers announced that, "Connie Fisher sang through a heavy cold, which has caused a vocal injury," and revealed that she was taking two weeks off. In fact a blood vessel had burst in her throat and her voice had gone completely. When she returned, a tabloid paper suggested that she was now miming her performance. In response, the producers explained that their star now had to use a "click track" to hit the top note in "I Have Confidence" in case she couldn't reach it.

"In my head I was superwoman," she said later. "I'd just won this show and I had to prove myself to everybody." There, perhaps, is the problem. Musical actresses who take on lead roles, eight times a week, for an extended run, used to be seasoned professionals who had gradually grown used to the emotional and physical strain of nightly performance. Fisher was the first instance of an aspiring singer who becomes an overnight theatre star without the rough experience of what a gruelling business it all is.

Her memories of the Palladium year, she has said, are of "nervousness, dread , exhaustion". She hated the media's intrusion into her relationship with her boyfriend, Neal Williams, a soldier.

Now, she has begun to embrace a world outside the theatre. She released a debut album called Favourite Things. She has sung at events such as Bryn Terfel's Faenol Festival in August. And she has signed up for her first television drama, an ITV thriller called Caught in a Trap, about a crazed Elvis fan. She has even begun talking to interviewers about doing Shakespeare, or something cutting-edge at the Royal Court.

The trouble is, it doesn't sound like Maria, any of it. Audiences know what they like, and they liked the pluckiness of the gawky girl-next-door with the heavenly voice.

Can it be that they don't want her to see her as anything else? They might have been happy with the idea of her in her new Simply Bernstein incarnation, singing selections from West Side Story in a posh frock – but they don't want to come and see her unless she's going to launch into "The Lonely Goatherd".

The next step? Lloyd Webber is considering casting her as the comedian Fanny Brice in Funny Girl, the role that put Barbra Streisand on the map. "Connie would love to play the role," he told The Guardian, "and she would make a wonderful Fanny Brice. Connie is a major star and I believe she's going to be around for a long time."

We all believed that in November 2006. Perhaps it's true. In the meantime, Fisher might note that the big number in Funny Girl is "I'm the Greatest Star". Might that be tempting Fate a little too much?