First method:

1. In order to switch off the device hold down the Power key for a short while.
2. Afterwards press and hold together Volume Down and Power button until you feel a vibration.
3. When you feel the vibration, immediately press and hold the Volume down button until you see a large exclamation mark.
4. Then press the following sequence of buttons: Volume Up - > Volume Down - > Power - > Volume Down.
5. Success! Tha hard reset has just been performed.

Second method:

1. If the display is off, press the Power to turn it back on.
2. In the next step go to Windows Phone Settings.
3. Afterwards select About.
4. To confirm the whole operation tap Reset your phone.
5. Well done! It may take a while to finish the hard reset.