What is wrong with Ophelia? What is the cause? How does she behave?

Back in the court of Denmark, we see Gertrude speaking with a gentleman who explains that Ophelia has gone mad. She is rambling nonsensically about her father and insisting on seeing Gertrude. The queen reluctantly admits Ophelia, who proceeds to sing a number of simple and haunting songs, some of them quite bawdy. Ophelia has nowhere to turn to. She has been used by all the men in her life. Her father is dead and Hamlet has rebuked her as a strumpet.