If your BlackBerry hasn't been working since Thursday you have another problem. There are no T-Mobile outages reported. Sorry to disagree with Sandy, but AT&T and T-Mobile are different and there aren't reports of T-Mobile outages.
Snep - have you tried standard troubleshooting - popping the battery out and in?
Is this a corporate or personal BB, and finally, what version of the OS are you using (Options / About will tell you).