HTC One M8 for Windows - Manual (Page 30)

Phone not responding?
Restart your phone if it doesn't respond when you touch the screen
or press the buttons.
Press and hold the POWER button for at least 12 seconds. Your
phone turns off and then turns back on.
Touch gestures
Use touch gestures to get around your phone.
Tap
Tap the screen with your finger when you want to
select onscreen items such as application and settings
icons, or press onscreen buttons.
30 Your first week with your new phone