Why doesn't my Iphone have service?

I got it off craigslist today. It is jailbroken so i can use it for tmobile and i put my sim in and it recognized it. But now i have no service.i've tried to reset the network settings and everything. I honestly have no idea what is wrong with it and i am so frustrated. Please help

It has to be unlocked. Jailbreaking has nothing to do with using a phone on another network.

Its likely been reported stolen/lost (most phones on craigslist are)

this means its put on an international blacklist, and its illegal for any wireless company to put it on their network.