Erin Moran, who played Joanie Cunningham in the sitcoms Happy Days and Joanie Loves Chachi, has been found dead.
Police in Harrison County, Indiana, said the dispatcher "received a 911 call about an unresponsive female" on Saturday.
"Upon arrival of first responders, it was determined that Erin Moran Fleischmann was deceased. An autopsy is pending," the sheriff's department added.
Moran, 56, a native of Burbank, California, who had been married to Steven Fleischmann, began acting in TV and movies before she was 10.
She had nearly a decade's worth of experience in 1974 when she was cast in Happy Days as Joanie Cunningham, the kid sister to high school student Richie Cunningham, played by Ron Howard.
"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."
Debuting at a time of nostalgia for the seemingly innocent 1950s, the sitcom was set in Milwaukee and soon became a hit.
Howard and Henry Winkler, who played tough guy Arthur "The Fonz" Fonzarelli, were the show's biggest stars, but Moran also became popular.
In 1982, she was paired with fellow Happy Days performer Scott Baio in the short-lived Joanie Loves Chachi.
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."
Speaking about her Happy Days times, Moran told Xfinity in 2009: "What happened with all of us was like we were this family.
"It was so surreal with all the cast members. They were my family, get it?"
"I would love to do a feature (film), I'd love to do a play," she told CNN in 1981 when asked what she would like to do after Happy Days.
In 2011 Moran, along with Marion Ross who played Joanie's mother 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.
Moran told Xfinity that she had been working on an autobiography, Happy Days, Depressing Nights.