The Queen's speech in act 4, scene 7 suggests that Ophelia's madness probably drove the young woman to her death.

In the speech, the Queen notes that Ophelia made a strange wreath to hang on the branches of a tree before she drowned. These particular combination of flowers are said to have a decidedly raunchy name. However, young women with good morals usually call them "dead men's fingers."

There with fantastic garlands did she come

Of crowflowers, nettles, daisies, and long purples,

That liberal shepherds give a grosser name,

But our cold maids do “dead men’s fingers” call them.

Well, rather than suggesting these things, some of what the speech says is quite direct: the queen says that Ophelia drowned because she could not bear her own "distress"—that she killed herself because she was too sad, and too upset.

The madness, though, is a more complicated question. This the queen links to a kind of transformation, as if by turning mad Ophelia was partially mythological and partially animal, like she's stuck between worlds. Like the ghost, in other words.