Airtel — Forced prepaid to postpaid

I had recharged my prepaid number [protected] on 3rd November 2020 i.e 399 56 days reacharge, after a while my sim stopped working, and as I talked to the coutomer care they informed me that my sim plan is converted from prepaid to post paid. And that's without any notice and me not even wanting it. And that I will be charged a bill at the end of the period. And my 399 prepaid recharge is collapsed (gone). I can't understand the process that how can a sim be converted from prepaid to postpaid during an ongoing plan?. And would like i[censored] look into this matter and get my sim back to prepaid.