Margot Kidder, the actress who played Lois Lane in "Superman," died Sunday in Montana ... TMZ has learned. 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 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...