Moran's body was found unresponsive on Saturday afternoon by authorities in Indiana. The cause of death is unknown, reported Variety.
Born in California, the actress also starred in the "Happy Days" spin-off "Joanie Loves Chachi".
Moran had fallen on hard times in recent years. She was reportedly kicked out of her trailer park home in Indiana because of her hard-partying ways.
Henry Winkler, who starred opposite Moran as The Fonz in ABC's iconic series, tweeted, "OH Erin...Now you will finally have the peace you wanted so badly here on earth. Rest In It serenely now. Too soon."
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."
Moran's TV credits also included "The Love Boat," "Murder, She Wrote" and "The Bold and the Beautiful.