the mobile network on my blackberry changed randomly from T-Mobile to AT&T?

i have a blackberry bold 9700 and my mobile network carrier is T-Mobile. I've had the phone for a couple months and it was working just fine. then i got an error that says "Insert SIM Card" and so i read online that i should check the mobile network options. So I checked it and my mobile network is listed as AT&T! what do i do?
 
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