A master reset can be useful convenient in when you need to sell your device (clear all personal data). It can also help you in situations where your device has become unresponsive, is experiencing major slowdowns or random crashes.
Resetting your phone will result in the lost of all your personal data such as messages, contacts, photos, applications and personal settings. We strongly recommend that you backup your data beforehand. You to synchronize your contacts and messages with your Microsoft account copy your photos, videos and documents on your computer or upload them on Onedrive.
Reset your device from the Settings menu
This operation can be performed from the settings menu of your device. From the homescreen, swipe left to display the App List and then go to Settings > About. Tap on Reset your phone and follow the onscreen procedure. If your phone is not responding and you are not able to access the settings, you can try resetting it with the buttons. But before that, try to force the reboot. In some cases, this can unlock your phone without resetting it.
Reset an unresponsive device
Force restart: Software reset
You can force restart your device if it has stop responding (app or system failure). Hold the Power button and Volume Down key for 15 seconds (until you feel a small vibration). You can release the buttons. Repeat the operation several times if it were to fail. If your phone doesn't support this feature, then removing and reinserting the battery will produce the same effect. After restarting your phone, you'll probably need to reset the time and date.
Force restart: Hardware reset
If the forced reboot did not solve the encountered issues, you can try to perform a hard reset. First make sure the device is off. Hold the Volume Down key and the press the Power key for a couple of seconds. You can release the Power key as soon as the Windows logo appears on your screen, but keep the Volume Down key pressed until you see a white exclamation. Press the following keys in the given order: Volume Up > Volume Down > Power > Volume Down.