please help

I let my friend use my galaxy note 2 for a while cause his phone was broke.... When I got the phone back I found out that he had had the phone flashed to use with T-Mobile,,,,, I did a update to the 4.1.2 but i'm still unable to get service on the phone.. was wondering if maybe the radio bands had been changed or something of the sort... please help if you can... I do get my at&t service in the house cause i'm around a at&t micro cell.... but as soon as I step outside all I get is a circle with a line threw it

 

I don't know how to flash it back to at&t.... and i'm new to this fourm thing and don't know how to pm

 

I think I just messaged you