Welcome to Android Central! Are you unable to reboot the phone? If it seems frozen, force a shutdown by pressing and holding Power for about 10-15 seconds. Then power back on, and see if the problem persists. If it does, try booting into Safe Mode instead:
https://support.t-mobile.com/docs/DOC-31470. If the problem disappears in Safe Mode, then something you installed is causing the problem. You may have to uninstall apps one by one until the problem disappears. If the problem persists in Safe Mode, then it's more likely to be a hardware or firmware problem.