Which phone is it? Every Windows phone has a hardware combination that allows you to manually hard reset the phone if need be. You might try this and see if it works. This is the method I know for Windows Phone 8.1 HTC devices. Someone can correct me if I am wrong.
Turn the power off.
Press and hold the VOLUME DOWN button, and then briefly press the POWER button. Release the VOLUME DOWN button when you see an icon on the screen.
Then press the following hardware buttons in sequence: VOLUME UP > VOLUME DOWN > POWER > VOLUME DOWN