Airtel — my sim is blocked and someone else is using my number

My sim is blocked for no reason and my phone number has validity and money to make a call. And someone has bought my sim number? I really don't understand how airtel can give my sim number to someone else while i am using it, i was unable to contact my old friends and i was unable to access my email and other login due to 2-step verification and i have been using this number more than 3 years. Really worst service from airtel.