Date of relief from employment is not mentioned.
Unpaid salary is a money claim. After one more reminder and request for payment, and if no payment is received still, a money suit can be filed in the jurisdictional court. Time for disposal of civil suits varies from court to court and place to place, generally 18 months to 3years.
Limitation for filing such a suit would be 3years from the date of relief.Court fee for amount claimed is payable to court which varies from state to state. Assistance of a lawyer is necessary, as it may not be possible for a lay person to withstand the hassles of attending the court on multiple dates.
A cost benefit analysis to be made depending upon the amount involved and lawyer's fees which is to be negotiated by you.
Non-dsclosure agreement does not come in the way for filing the suit.