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. In spite of many attempts, the 'script' has not yet been deciphered, but efforts are ongoing.
Source : Wikipedia
The Indus civilization existed from about 3300 BCE to 1300 BCE. It then mysteriously declined.(see :Indus civilisation collapseplugin-autotooltip__plain plugin-autotooltip_bigIndus civilisation collapse
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.)
It covered at least 800,000 square kilometres of what is now Pakistan and Northwestern India, and had a population of around one million.
Scripts from the civilization were first described in 1875, and since then details of more than 4,000 other examples have been published.
The number of individual 'signs' in the scripts have been estimated from just 63 up to 676.
The scripts have been extensively studied, with many (conflicting) attempts at deciphering their meaning.
Whatever their differences, all Indus researchers agree that there is no consensus on the meaning of the script.
Further info Nature 526, pages 499–501