Hello,
1) Was the signed offer letter addressed to you on person on the company's official letter head?
2) Had you received written instructions from the Employer to resign from the perspective Employer?
3) If the answer to the above questions are in the affirmative, you need to send them an e-mail stating that you will be constrained to take legal recourse of the company failed to employ you as offered or in the alternative adequately compensate you for your loss.
4) If you still fail to get any response contract a lawyer and get a legal notice sent to the company.
5) In the meantime do some research about the authenticity of the company and if they have any offices in India. This will help.