The Indus Civilisation (a.k.a. the Harappan Civilisation) was a flourishing Bronze Age civilisation in the northwestern regions of South Asia, lasting from around 3300 BCE to 1300 BCE.
The civilisation, which was highly advanced and substantially urban, is thought to have quite suddenly collapsed.
Excavational records show that around 1900 BCE, signs of a gradual decline began to emerge, and by 1700 BCE the five carefully planned urban centres appear to have been largely abandoned.
Droughts, floods, tectonic shifts, ideological turmoils and foreign invasion have all been invoked to explain its spectacular collapse"
Source : Science Vol. 320, No. 5881
Other scholars suggest that diseases may also have played a part.
There is still no general academic agreement over what caused the collapse.
Further reading see : Wikipedia
Also see : Indus scriptplugin-autotooltip__plain plugin-autotooltip_bigIndus script
"The Indus script (also known as the Harappan script) is a corpus of symbols produced by the Indus Valley Civilization. Most inscriptions containing these symbols are extremely short, making it difficult to judge whether or not these symbols constituted a script used to record a language, or even symbolise a writing system.