Use this option if your device is locked and you don't know the code.
Note: Make sure that your Nokia Lumia is turned off.
Step 1. Press and hold the power key until it vibrates then release.
Step 2. Press and hold the down key volume. If the exclamation mark will be shown on the screen, you can now release the button.
Step 3. You need to input these key combination:
Volume Up > Volume Down > Power > Volume Down.
Your phone will reset and will boot up automatically. This might take a several minutes