Learned members/experts have given valuable suggestion. Kindly follow it.
You must be having the acknowledgment of submission of notice of resignation and you must have mentioned effective date of resignation in it. You have served the notice period, and thus have completed your part of obligation.
You must have submitted company property and completed tasks on hand, must be having acknowledgment.
If you do not have then you should submit a written communication confirming the same and thus build record in your favor.
You can have a heart to heart meeting with your reporting authority, HR and apply your rapport, goodwill, persuasions, persistence, negotiation, skill to settle the matter in your favor, amicably.
No one can force or coerce you to continue to be in employment against your wishes.
You can submit a written communication under acknowledgment addressed to good offices of your appointing authority, MD, HR Head, Company secretary and mention date of submission of your notice of resignation, effective date of resignation, submission of company property, completion of all tasks, and ask their good offices to intervene and to supply you the acceptance of your resignation, work experience/service certificate, form 16,PF accumulation reports and withdrawal/transfer forms, relieving letter ,NOC/NDC,FNF statement, settlement of dues and payment of dues say within next 7 days, etc. by regd/speed post.
You can highlight your achievements and that as a good employee you have submitted notice and served notice period and good offices should ensure smooth exit and should supply you all docs, certificates, payments and ensure that nothing is delayed and your future ventures employability, record is not affected. Company can sit on relieving letter and can not sit on others. Other documents/records can serve to be as good as relieving letter.
If you do not get relief you can lodge complaint with o/o labor commissioner or approach your lawyer.