As per your statement , it seems that you have stopped providing services and hence you are not liable to charge service tax consequently there is no collection of service tax .
hence in this you are not liable to file the service tax return . However in this circumstances you should surrender your service tax registration . Further you have also give in writing to the department that you are no longer providing any services and not collecting any tax and hence not filed the return .
You should be able to prove through your bank statement or ITR that you are no longer providing the services .
Thus to conclude , please apply for surrender of service tax registration and submit a written statement to the department about your present status and keep acknowledgment of that in order to defend yourself .