Airtel — Prepaid to postpaid without any notice or will

I had recharged my prepaid sim on 3rd nov 2020 of rs 399 56 days. Later that day I had a network issue and my sim stopped working. I contacted the customer care they informed me that my sim service has been switched to postpaid and that was done by me. And I am damn sure it wasn't. They also said that my 399 prepaid recharge is collapsed (gone) and that I will be charged a postpaid bill at the end of the month. This is completely my loss and it's not even my fault. I want my sim service back to prepaid as soon as possible
Please look into this matter as this is highly inconvenient.