Rest in peace, Margot Kidder. The actress — who played Lois Lane in the Superman films — sadly died on Sunday, May 13, from unknown causes, TMZ first reported. She was 69 years old.

Margot passed away at her home in Livingston, MT, and funeral arrangements for her have currently not been set yet. "Margaret Ruth 'Margot' Kidder, 69, of Livingston, MT. The actress and activist passed away on Sunday, May 13, 2018 at her home. Arrangements are pending under the care of Franzen-Davis Funeral Home and Crematory in Livingston, MT," the funeral home's site read.

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Margot kicked off her acting career in the 1960s, but rose to fame in 1978 when she starred alongside Christopher Reeve in Superman: The Movie. She reprised her Lois Lane role in the movie's three sequels as well.

Some of Margot's other acting credits include shows Captain Planet and the Planeteers, Shell Game, Phantom 2040, Nichols and Boston Common, as well as movies The Amityville Horror, Heartaches, and Black Christmas.

However, Margot's career took a turn for the worse in 1996 when she suffered a mental breakdown. "The woman we saw was in obvious mental distress," Officer Rick Young of the Glendale police told People after she was missing for days. "She didn’t think one person was following her. She thought a whole group of people were after her."

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Soon after, Margot revealed to People that she was battling manic depression and suffering "mood swings that could knock over a building." She added, "It’s very hard to convince a manic person that there is anything wrong with them. You have no desire to sleep. You are full of ideas."

After the episode over 22 years ago, Margot's career kicked back off with roles in shows including Smallville, Law & Order: SVU, and more. It's unclear if her passing was related to her mental health.

Our thoughts are with her family and friends during this difficult time, especially her daughter, Maggie McGuane.