Definite reason that led to the decline of the Indus Valley Civilization is not known, as no reliable resource of that period is available at present. Every conclusion regarding the decline is based upon speculations of historians. Though the reason of the decline is not known, but through excavations it is clear that fall of the Harappan Civilization occurred between 1800 BC to 1700 BC.
1) aryans from central asia invaded harapans so, the harappans migrted
2) the course of indus river was changed so the whole civilization migrated with the river
3) may be some natural disaster such as flood or drought may had occured so, the people in civilization had disappeared