The actress's cause of death is unknown.
Her career was erratic after the Superman films as she publicly battled bipolar disorder. She was especially outspoken as a mental health treatment advocate in the late 90s.
We all have our favorites when it comes to actors and actresses who play iconic characters in the movies and TV. Despite the fact that I grew up watching the Superman television show with George Reeves as Superman and Noel Neill as Lois Lane (later replaced by Phyllis Coates), Margot Kidder is MY Lois Lane. And, with apologies to Iowa's own Brandon Routh, Christopher Reeve IS Superman.
Do you remember all the hype around the first Superman movie with Reeve, Kidder, and Marlon Brando? "You will believe a man can fly." I was actually a player in a school basketball tournament and the entire team was taken to the movie following a day of games. The critics weren't so kind to the movie at the time (I think the expectations were TOO high), but to a kid watching the origin story play out on the big screen, it was magic.