In Hamlet, by William Shakespeare, Ophelia is the daughter of the king's counselor, Polonius, and the woman Hamlet loves. Whether or not Hamlet's love for Ophelia is real, it is clear that Ophelia loves Hamlet. Two things happen that have a dramatic impact on Ophelia's life.

First, her father is killed. As if that were not bad enough, he is accidentally killed by Hamlet, the man she loves. Her father's death would have been traumatic, of course, but the fact that Hamlet did it makes her even more distraught.

The second thing that happens is that Hamlet is sent away by the king, leaving Ophelia virtually alone in her grief, since her brother, Laertes, is back at school.

The result is almost predictable: Ophelia goes mad.

In Act IV, scene v, she appears before the queen, and it is clear that she has lost her senses. Laertes has returned for his father's funeral, and he observes his sister, fueling the fire of his anger toward Hamlet. 

In Act IV scene vii, Gertrude announces to Laertes and Claudius that Ophelia had drowned. She was perched on the bough of a tree with all her flowers when the limb broke and deposited her into the water. 

... Her clothes spread wide;
And, mermaid-like, awhile they bore her up:
Which time she chanted snatches of old tunes;
As one incapable of her own distress,
Or like a creature native and indued
Unto that element: but long it could not be
Till that her garments, heavy with their drink,
Pull'd the poor wretch from her melodious lay
To muddy death.

Ophelia drowned because she fell into the water and did not have the will, in her grief,  to save herself.