THE MYSTERY OF THE END
Around 3900 years ago we find the beginning of a major change. People stopped living in many of the cities. Writing, seals and weights were no longer used. Raw materials brought from long distances became rare. In Mohenjodaro, we find that garbage piled up on the streets, the drainage system broke down, and new, less impressive houses were built, even over the streets.
Why did all this happen? We are not sure. Some scholars suggest that the rivers dried up. Others suggest that there was deforestation. This could have happened because fuel was required for baking bricks, and for smelting copper ores. Besides, grazing by large herds of cattle, sheep, and goat may have destroyed the green cover. In some areas there were floods. But none of these reasons can explain the end of all the cities. Flooding or a river drying up would have had an effect in only some areas.
It appears as if the rulers lost control. In any case, the effects of the change are quite clear. Sites in Sind and west Punjab (present-day Pakistan) were abandoned, while many people moved into newer, smaller settlements to the east and the south.
New cities emerged about 1400 years later. The emergence of other cities (about 2500 years ago).
Source: This topic is taken from NCERT TEXTBOOK