Airtel — Airtel postpaid sim not working

It has been more than a year that i am using airtel postpaid. But recently my postpaid sim just stopped working out of nowhere. The sim shows no network. I tried putting it in many different phones but same result whereas my prepaid sim works just fine. I am not able to reach to the customer care. Whenever i call them an automated robot registers the complaint. My complaint is registered since a month but no resolution yet. I am posting 2.1k each month without using anything. When i visited a nearby store they said that they couldn't do anything.