What should I do if someone denies repaying borrowed money?

Sir, someone borrowed money from me and assured me that they would definitely return it. However, now they are denying the full amount. They claim the total amount is approximately ₹36,000, but the actual amount is ₹1,37,000 (which is my principal amount). I only have their signature on a normal piece of paper as proof. The money was borrowed on 15th February 2017. What should I do? Please guide me.