Erin Moran, the former US child star who played Joanie Cunningham in the hit sitcom Happy Days, has died aged 56.

A statement from the sheriff’s department in Harrison County, Indiana, confirmed the death and said an autopsy was pending.

It said a dispatcher received a 911 call about “an unresponsive female”, and Moran was found dead when medics arrived.

A Burbank, California native, Moran began acting in TV and movies before she was 10 years old.

She had nearly a decade’s worth of experience when she was cast in 1974 in Happy Days as Joanie Cunningham, the kid sister to high school student Richie Cunningham, played by Ron Howard.

Other cast members included Tom Bosley and Marion Ross as Joanie’s parents and Henry Winkler as the loveable tough guy, Arthur ‘The Fonz’ Fonzarelli.

“What happened with all of us was like we were this family,” she told Xfinity in 2009.

“It was so surreal with all the cast members. They were my family, get it?”

“Such sad, sad news. RIP Erin,” Howard tweeted.

“I’ll always choose to remember you on our show making scenes better, getting laughs and lighting up TV screens,” he said.

Erin Moran was cast as Joanie Cunningham in Happy Days in 1974. Photo: AP

Debuting at a time of nostalgia for the seemingly innocent 1950s, the sitcom was set in Milwaukee and soon became a hit.

Howard and Winkler were the show’s biggest stars, but the smiling, freckle-faced Moran also became popular.

In 1982, she was paired off with fellow Happy Days performer Scott Baio in the short-lived Joanie Loves Chachi.

Moran returned to Happy Days in 1984, the show’s final season.

Her more recent credits included The Love Boat and Murder, She Wrote, but she never approached the success of Happy Days and was more often in the news for her numerous personal and financial struggles and was reportedly homeless at times.

“Oh Erin… now you will finally have the peace you wanted so badly here on Earth,” Winkler tweeted.

“Rest in it serenely now … too soon.”

In 2011, Moran and Ross and other former Happy Days actors Anson Williams and Donnie Most sued CBS, saying they were owed money for merchandising related to the show. The lawsuit was settled the following year.

Moran told Xfinity that she had been working on a memoir, called Happy Days, Depressing Nights.

Moran was married to Steven Fleischmann.

OH Erin… now you will finally have the peace you wanted so badly here on earth …Rest In It serenely now.. too soon

— Henry Winkler (@hwinkler4real) April 23, 2017

Such sad sad news. RIP Erin. I'll always choose to remember you on our show making scenes better, getting laughs and lighting up tv screens. https://t.co/8HmdL0JKlf

— Ron Howard (@RealRonHoward) April 23, 2017

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