This post will help you solve a continuous reboot issue with HTC 8X. In this problem, the phone won't be able to go pass the "HTC Quietly Brilliant" screen. It keeps rebooting itself on and on and on and it will not POWER OFF. The HTC 8X has a non-removal battery and so the phone cannot be switched off by removing the battery.
BTW, I got this issue, in a brand new box packed HTC 8X.
The solution, which worked for me was a FACTORY RESET of the phone. For doing a Factory reset, the HTC Manual gives the following steps -
The other way to reset is by using the phone buttons, which will be applicable in this case knowing the fact that the phone can't be turned ON or access the setting screen.
Here is the "!" on me phone (Whew! it took me 6 attempts to get to this screen)
Post that, the phone will reboot once and you should see the following screen:
I fixed another phone for my friend. On his phone, I got these screen shots during retries –
Just reboot once more if you get such a screen and attempt to get that "!" icon.