The Harappan civilization finally declined between 2000 and 1800 BC. We cannot really pinpoint one single reason. Climate changes may have played a role, since the Earth was becoming warmer during this time. The land became drier, and water ran short for cultivation.
It is also thought that earthquakes or fires may have destroyed many cities. The city of Mohenjo-Daro shows evidence of having been destroyed by fire. Other scholars feel that the population was wiped out by an epidemic or disease. It is also likely that the area became too large to be effectively controlled, encouraging attacks by outsiders. Some believe that the nomadic Aryans invaded India around 1500 BC, and destroyed the Indus Valley civilization, bringing to an end the most brilliant civilization of the ancient world. Many modern historians say the theories of Aryan invasion are a myth.