We're told authorities in Indiana got a call just after 4 PM ET Saturday from someone reporting an "unresponsive female." EMT's arrived and found Erin Moran's body ... she was already dead.

Moran shot to spotlight in the early 70s when she was cast on "Happy Days" as Joanie, the younger sister of Ron Howard's character. She continued the role in 1982 alongside Scott Baio in "Joanie Loves Chachi" but the show only lasted one season.

Erin had a tough time after her "Happy Days" stardom ... a combination of drinking and bizarre behavior that eventually landed her in a trailer park in Indiana. She had run out of money and was reportedly kicked out of the mobile home she shared with husband Steve Fleischmann.

Through her career, Moran also landed roles on "Murder, She Wrote," "Desperation Boulevard," and "Celebrity Fit Club."

An autopsy is pending.

She was 56.