Why is my Android update freezing?

[AC Question]

Android updating freezing

I've tried to update my HTC M8 with the latest version. The updated downloaded but when I chose to install it I got a reboot screen and now it's stuck on the white screen you get when you turn the phone on. I've tried rebooting my holding the on/off and volume up button but that just goes around in circles and freezes again. I've also tried the on/off and volume down button but that doesn't do anything. Help!