If you are askking about the story; She felt like she was captivated because of her marriage and her oppresive husband. When she learned that he died, she thought after all those years she was finally free. And when she saw her husband at the door, all this joy disappeared because her husband was back and she wasn't free anymore. She had a heart attack ouf of shock probably. But I think it might be because of overdose of sadness.

I agree with Seda's answer with the exception of the word "captivated". Captivated means to be enchanted, enthralled. :) The protagonist felt as if she was being stifled by her husband and society's expectations. Before she heard about her husband's "death" she was concerned that life might be too long. Obviously she welcomed any wscape. Considering heart condition and her dread of living this way, her heart attack and sudden death should not be a surprise. What is ironic is how the other characters and society view her heart attaxk. What was the wording? She died from joy? Or something like that.

*escape *attack *her heart condition

I think the reason for Mrs Mallard's death was that when her husband came back she knew that she was no longer free.