[Update on 31 Dec 2013 - video link added at end of the blog post]
[Update on 10 Feb 2014 - 2 issues found when the below fix won't work]
Few weeks back, I tried the same method on 2 HTC devices and I could not apply the fix. There were 2 interesting issues found -
1. Super-fast reboot
In this case, the white HTC start screen would show only for 2-3 seconds and then the phone would reboot. And this happened only when the phone was connected to the charger. The moment, the charger was removed, the phone would be dead as a rock.
2. Missing "RED" light during charge
In this case, the reboot worked correctly and the white HTC screen would stay for a good 15-20 seconds. However, when I plugged the phone with the charger, the "RED" charging indicator on the top right area of the phone DID NOT TURN ON (see screen shot below). I tried with few different charges and wires, but the RED LED did not turn ON during charge. I tried the factory reset combination on this phone, but could not get to the "!" icon (see the solution below). HSo, in case you have the above 2 issues of super-fast reboots and the missing "RED" during charging, then it is possible that the below solution might not work.
[Update on 10 Feb 2014.... ]
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.
[Update on 31 Dec 2013]
I have recently uploaded a video on YouTube on how to do a factory reset on HTC 8x and get the “!” icon. I could not re-create the reboot problem on my phone, but have tried explaining the concept on how you can get the ! icon and set the timings, which is the key to do a factory reset.
Link - http://www.youtube.com/watch?v=LohzuvDtB7M