Margot was at her home when she passed away, according to the spokesperson at Franzen-Davis Funeral Home in Livingston, MT. The cause of death is unknown at this point.

Bruce Becker, the Park County Attorney, tells TMZ ... an unknown person called police to report Margot was unconscious and not breathing. Police responded and found her dead. The Livingston Police Chief says foul play is not considered a factor, but her death is now under investigation.

Margot starred opposite Christopher Reeve in 1978's "Superman," and also in the 3 sequels. She continued acting right up until this year, taking small roles in TV shows and movies -- but also had stage roles ... including "The Vagina Monologues" on Broadway. She reportedly had 2 more roles yet to be released.

Margot had a very public battle with bipolar disorder for years. During her struggle she was briefly homeless in 1996. Margot became an advocate for mental health after that incident.

She'd been married 3 times, although she'd been single since the '80s.

Margot is survived by a daughter.