Showing "No Service" on my prepaid mobile No 9896920081

I have a prepaid Airtel Mobile No 91 9896920081. I had to come USA as a visiter form some months. It was conected to AT&T when arrived in USA for 2-3 months it remained connected but now it is indicating NO SERVICE.  Suresh Chander kaushik sckaushik_2000@yahoo.com