You basically die. Not on that particular amount, but rather by excessive amounts. Anything, in high enough quantities, becomes a poison.
But since you asked for details, an over-consumption of water dillutes the electrolytes which ends up generating brain-cell swelling, increasing intercranial pressure. So, you start getting symptoms from headaches to personality changes. Then it gets worse (with a lot more symptoms) until the pressure clogs your own brain and you die.
At the onset of this condition, fluid outside the cells has an excessively low amount of solutes, such as sodium and other electrolytes, in comparison to fluid inside the cells, causing the fluid to move into the cells to balance its concentration. This causes the cells to swell. In the brain, this swelling increases intracranial pressure (ICP), which leads to the first observable symptoms of water intoxication: headache, personality changes, changes in behavior, confusion, irritability, and drowsiness. These are sometimes followed by difficulty breathing during exertion, muscle weakness & pain, twitching, or cramping, nausea, vomiting, thirst, and a dulled ability to perceive and interpret sensory information. As the condition persists, papillary and vital signs may result including bradycardia and widened pulse pressure. The cells in the brain may swell to the point where blood flow is interrupted resulting in cerebral edema. Swollen brain cells may also apply pressure to the brain stem causing central nervous system dysfunction. Both cerebral edema and interference with the central nervous system are dangerous and could result in seizures, brain damage, coma or death.
