|Erin Moran pictured in 2003|
Police in Harrison County, Indiana, said the former child star was found unresponsive after authorities received an emergency call. The cause of her death has not been determined.
Californian-born Moran began acting in TV and movies before she was 10 years old and joined the cast in Happy Days as Joanie, the kid sister to Ron Howard's Richie Cunningham, in 1974.
Her other co-stars 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," Moran told Xfinity in 2009. "It was so surreal with all the cast members. ... They were my family, get it?"
The show was set in 1950s Milwaukee and became a long-running hit. Howard and Winkler were the show's biggest stars, but the smiling, freckle-faced Moran also became popular.
|Happy Days' cast members: Marion Ross, centre, director Garry Marshall, left, Tom Bosley, Erin Moran, Henry Winkler and Anson Williams|
In 1982, she was paired off with fellow Happy Days performer Scott Baio in the short-lived spin-off Joanie Loves Chachi.
Moran returned to Happy Days in 1984, the show's final season.
On news of her death, Howard tweeted: "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."
"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."
Moran's more recent credits included The Love Boat and Murder, She Wrote, but she never approached the success of Happy Days and endured numerous personal and financial struggles. She was reportedly homeless at times.
In 2011, she and Ross and 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.