The Indus Valley Civilization collapsed during 1500 B.C.E., shortly after the decline of the major cities of the valley: Harappa and Mahenjo-Daro.
Though there is not an exact reason given for the decline of Civilization, numerous researchers have suggested a few reasons behind it.
The invasion of Aryans being the major one where the Aryans are believed to have migrated for the sole purpose of invading the advancing urbanization and eyeing to take over. They came and started farming along with doing various trading.
Another major cause proposed by the researchers is the change in the climate, which caused the Saraswati River to dry, disastrous change in eastward-moving monsoons and also due to a massive flood.