My first guess would be a bad battery.
Take it into a TMo store, or a Radio Shack that carries T-Mo phones, and ask them if they can pop in a new battery to see if it works. If it does, replace the battery and your problem is solved. If your phone is less than a year old, the battery itself is under warranty, and should be replaced for free.
If not, maybe the tech dept (if your store has one) might be able to take a quick look at it and give you a diagnosis.
Good luck!
