Answer: Indus Script
Step-by-step explanation:
You are most probably referring to the Ancient Indian civilization of the Indus Valley which existed from around 3000 BC to 1300 BC. The Indus Valley Civilization was quite advanced for its time as it had urban planning and drainage systems as well as water supply amongst others.
They had a written language called the Indus Script which sadly has proven difficult to translate due to the lack of a translation medium like the Rosetta stone for ancient Egyptian hieroglyphs.