Press and hold the Volume down Volume down button and Power Power button buttons at the same time until you feel a vibration (about 10–15 seconds).
When you feel the vibration, immediately press and hold the Volume down Volume down button button until you see a large exclamation mark.
Once the exclamation mark appears, press the following four buttons in this order: Volume up Volume up button, Volume down Volume down button, Power Power button, Volume down Volume down button. Your phone should now reset and restart itself. (It might take a while for the reset to finish.)