The first thing to do is to check and see if it is the wall unit. Try the cable (Or better yet, a different cable, if possible) in a USB port of a PC, and see if it charges then. If it does, and you're able to get the phone to come on after charging, then you're all good.
If using a PC and a different cable still get you the same result, then it's possible you either have a bad battery, or something has broken on your phone itself. As long as you bought your phone new (And not from some other person), and it hasn't been dropped or gotten wet, then it is still under warranty. You can call T-Mobile customer care for a replacement. They will want to replace your battery first, and if that doesn't work, then they'll replace your phone.
If you live near a T-Mobile store, I'd suggest going there, and trying a replacement battery to see if it fixes the problem. That way, you don't have to wait for them to send you one, only to discover that it was the phone and not the battery.