If you ever need to hard reset your Windows Phone smartphone, here is a quick guide. note that a hard reset wipes out all your files stored on the device, your installed apps and your settings. be sure that is what you want to do before proceeding.
1. Hold down the Volume Down + Power + Camera buttons at the same time until the phone vibrates.
2. Once the phone starts to vibrate, release the power button, but continue to hold the other two buttons for about 5 seconds.
3. Phone reboots.
You have a clean device all over again.