I'm assuming that you are refering to Ophelia? She goes mad because of the treatment she receives from Hamlet. He is very abusive toward her by stating that he never loved her, he questions her virginity, calls her a whore in so many words. He responds to her with anger and sexually offensive remarks when she attempts simple conversation, like during "the mousetrap" scene.
Ophelia is usually seen as a weak character and is easliy ordered around and walked over by the men in her life. Hamlet is the most obvious abser, but her father defintely plays a part in her madness. He manipulates her to spy on Hamlet, even though she loves Hamlet and doesn't want to hurt him.