Dear Jignesh,
Under the provisions of the Indian Contract Act of 1872, an appointment letter is a contract between an employer and an employee. Since it is a contract, its provisions are binding on both parties. However, is the offer letter also a contract under the provisions of the said act? This is a moot point. Even if you file a suit against the company, and the judge rules that the offer letter is not a contract, your case could be defeated.
Furthermore, was there a clause on the liabilities for the revocation of the offer letter? If this clause was not mentioned, then on what grounds will you file a lawsuit?
Occasionally, the candidates accept the offer letter and later then back out. To avoid this last-minute no-show, the companies have started inserting the revocation clause as well as the clause on "no-show". This is called "Liquidated Damage (LD)". In your case please check the offer letter properly for the LD clause.
Notwithstanding what I have written above, I empathise with you. In life occasionally we do get setbacks. Take this as a setback and move on. Start searching for a job vigorously.
In the meantime, request the HR professional to send you a proper letter on the revocation of the job offer. Explain to him that the revocation of the job offer will create an employment gap in your career. To explain this gap, a letter on withdrawal of the job offer will come in handy.
Thanks,
Dinesh Divekar