Theory:

The Indus Valley Civilization, which flourished in modern-day India and Pakistan, started to decline around $$1800 BCE$$. This culture, as well as its two great cities, Harappa and Mohenjo-Daro, gradually vanished.

Harappa is the name given to the Indus Valley people because it was the first city discovered in $$1922$$by archaeologist R. D. Banerji.

R. D. Banerji - officer in the Archaeological Survey of India.
Archaeological findings led archaeologists to believe that trade with Mesopotamia had ceased in modern-day Iraq. The great city's drainage systems and baths were blocked.

Important!
Indus Script: These ten Indus Script symbols were found on a “sign-board” in the ancient city of Dholavira.
Writings began to disappear. The standardised weights and measures used for trade and taxation fell out of use.

Scholars have proposed various hypotheses to explain the disappearance of the Harappans, including an Aryan invasion and climate change characterised by massive monsoons.
Reference:
https://www.harappa.com/sites/default/files/question/mohenjo-daro-8-2.jpg