Archeologists have suggested many theories to describe the reasons why and how the Harappan Civilisation disappeared.
There might be two main factors for Harappans disappearance:
1. First factor was climate change which caused drought due to less rainfall.
2. Second factor was drying up of Sarasvatl river, again due to reduced rainfall.
3. Both above factors led to drought with less agriculture, reducing food supply to the cities.
4. There might also have been a pandemic (may be caused by severe drought) with breakout of a disease causing huge number of deaths at that time.
There could have been other factors, but these two remind us of how much we depend on climate and the environment for our well-being.