What legal action can I take to recover my loan?

I had given Rs. 3,00,000 as a loan with interest to a person who promised to repay my money along with interest by the stipulated date. The borrower and his wife had given an undertaking in writing on plain paper that if the loan was not repaid, their house would become my property. However, he has not repaid my money, and the stipulated date passed 2 years ago. He, along with his family, has shifted to an unknown location, locking the house. Please advise me.