The opening line of Kate Chopin's "The Story of an Hour" reveals the cause of Louise Mallard's death--heart trouble. Given the weakened state of her heart, Louise could not physically handle the news of her husband's "death."
Ironically, it is not the news of his death that succeeds at stopping Louise's heart. Instead, it is Brently (her husband) walking into their home after she has been told he had died in a train accident (or "disaster" as the text calls it).
Upon hearing about Brently's death, Louise locks herself in her bedroom (against the wishes of Josephine, her sister). Once in the bedroom, Louise realizes that she will finally be able to be free of the oppressive nature of her husband (illustrated by the "fair, calm face, whose lines bespoke repression"). Looking at Brently's death as the beginning to her new life, Louise opens the door, descends the stairs ("like a goddess of Victory"), and falls dead at the sight of her husband alive.
The irony behind her death lies in the fact that she does not die because of the grief she should feel at the news of her husband's death. Instead, she dies because she cannot handle the thought of having her new found freedom (""Free! Body and soul free!" ) ripped away. Her heart simply could not stand the thought.