Enforcing Bond and Relieving Issues

Dear Sir/Madam, I have been working in a Chennai based MNC for the past 6 years. After two years of joining, I have been sent abroad for training purpose (but handled live projects and not much training). Before departure, I have signed a contract with the employee stating that, I have to serve 3 years after my return to Chennai or if breached, a sum of Rs. 10 Lakhs to be paid as compensation. The contract was in just plain A4 sheet and not in a stamp paper. I duly signed and went to abroad. After two years I returned, I served 18 months in the same company, but got a better offer from another company, which is a competitor. I submitted my resignation, but my boss and HR did not agreed. So I formally sent an email after serving my notice period and left the company without any kind of relieving letter and settlements. Now I am working with the competitor. I received an email from the previous company HR stating that, they came to know I work with a competitor and asked me to confirm if this is true or not. If not replied in seven days, they would take necessary actions. I am worried and need legal advice on how to proceed further. What sort of things will I have to face and I am very much afraid that, I may spoil my name and carrier in the current organization. Please advice me on how to proceed and its quite urgent, since I have only 7 days to reply. Thank you.