Blanche went spiraling down when three major life-changing events came almost one after another, leaving her in limbo without knowing how or why it all happened.
The first was discovering her husband (whom she loved immensely) in bed with another man. Blanche could not bear it, and undestandably so. She thought she could shrug it off and move on- but she couldn't.
The second was her husband's sucicide. Given that Blanche was unable to shake off what she saw, she went out with the men (her husband and her lover) and, in a daze of alcohol she exploded. She told him "You make me sick", and this prompted his shooting himself. She carried that throughout the play, whenever the lights hit, and the music begins.
The third was the loss of Belle Reeve. No matter what anyone says, money CAN provide some comfort from stress and pain. She lost that too. When her father began to get sick already Stella had gone off to marry, and it was Blanche who stayed behind letting it all go and not knowing how to fix it.
All these things happened in a way that nobody saw coming. Blanche was not psychologically nor socially prepared to handle the situations. Her world, as she knew it, was removed from under her feet, and she became numb- only that, instead of moving above and beyond the problem, she succumbed to her inner demons: alcohol, sex, and debauchery.
Blanche is the epitome of the broken woman: One whose situation is so desperate that her former self dies and a new one has to be made up from the shambles in which she fell.