The first thing I would do it pull battery or do a soft reset. If that doesn't work, get a new sim card.
I agree with je_25, that's a good starting point to help out with this. You could also check out our Not connected to 3G, 4G, or LTE page. It's got other steps you can take and some important coverage information you'll want to take a look at to help get you connected.
Manage your T-Mobile account