Your case needs to be handled under the provisions of the Indian Contract Act (ICA), 1872. Since you are/were not an employee, it cannot come under the purview of labor laws.
Under the provisions of the ICA, an appointment letter is a contract between the employer and the employee. However, you were issued an offer letter and not an appointment letter. Therefore, the contract between the two parties was not legally established. Nevertheless, the offer letter is provided to enter into the main agreement. Should it also be construed as a contract? The answer depends on how the court interprets the law.
Section 4 of the ICA, 1872 deals extensively with the acceptance of an offer, whereas Section 5 deals with the revocation of the offer. The primary consideration is whether you had accepted the offer letter, how you accepted it, when the other party came to know about the acceptance of the offer, and after what time span the other party revoked it. Did you suffer any financial losses because of the revocation of the contract? If yes, do you have evidence?
I recommend you approach a lawyer who handles cases under the ICA and explain your case. Do not approach lawyers who handle labor cases.
Thanks,
Dinesh Divekar