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It is probably frozen. You can force reset it. Press and hold down the Power and Volume Down button for 10-20 seconds and this will force reset it.
If that didn't work, then connect it to your computer with the USB cable and then hold down Volume Down and Power.
Please let me know if that didn't work, so we can continue troubleshooting
If the above doesn't work, then remove the battery. Wait 2 minute. Put it back and then turn it back on. See here --> https://support.t-mobile.com/docs/DOC-21719