Misa’s will to live was solely dependent on Light’s existence. Her obsession with Light and his mission was one of her motivations to keep going. After Light’s death, she saw no reason to live and fell into deep despair. She believed that she had nothing left to live for, and her loneliness fueled her thoughts. In the manga, she died of suicide slightly over a year after Light’s death. However, her death wasn’t shown in the anime, but it is strongly implied that she died by jumping off a building. You’ll notice in episode 25 Misa was standing on the ledge of a tall building, looking at the horizon.