HTC One M8 User Manual page 119

119 Update and reset
Performing a factory reset using phone buttons
If you can't turn your phone on or access settings, you can still perform a factory reset
by using the phone's buttons.
1. Turn the power off.
2. Press and hold the VOLUME DOWN button, and then briefly press the POWER
button. Release the VOLUME DOWN button when you see an icon on the
screen.
3. Then press the following hardware buttons in sequence: VOLUME UP >
VOLUME DOWN > POWER > VOLUME DOWN.