We are not sure about what caused the end of the Harappan civilization.
Many historians do not believe in the Aryan Invasion theory. They believe the main cause of the sudden decline is climate change. Huge climate change or natural calamities might have been a cause of the ruin of Harappan civilization.
These changes in climate led to floods in plains and cities.
This civilization is said to have come to an abrupt end. The following reasons are put forward for its abrupt end: The neighboring desert encroached on the fertile area and made it infertile. Aryan invaders killed people and destroyed the Indus Valley Civilization.
End of the Indus. The Indus Valley civilization began to decline between 1900 and 1800 BCE. Most of the cities disappeared or were abandoned. Archaeologists do not know why this happened.