What's wrong with my at&t service? help!!!!!!!!!!!!!!!!!! 10 points for best answer!!!!!!!!?

I have a blackjack phone, and my service is acting very weird. It keeps saying i have no service and when, i tried to put my mom's sim card in the phone, it worked just fine. My service has done this before, but the details are a little different. It worked just fine last night, but this morning, its not working at all. I tried taking out the battery and putting it back in too. does anybody know whats going on????????? help!!!!!!!!!!!!