Boot loop after updating HTC One M8 to Android 6.0 Marshmallow Couple of days back, I got the notification to download and install official Android 6.0 (build 6.12.401) in my HTC One M8 (not rooted, was running Android 5.0.2). As I downloaded and installed it, from that point it's gone to boot-loop state and showing only HTC logo.

Now I have tried to do Factory reset that ends in the same loop, recovery mode which does nothing and stays on that screen with exclamation sign, installed ADB on a friend's advice, but the mobile's debugging is not on. 

Any advice or explanation of the issue and the fix?
Mine does this occasionally, the temp solution I have found for the time being is to plug it into a charger and then to force a reboot again by holding power and volume up together until it restarts. 

This normally gets the phone to boot, no idea why it makes any difference but it seems to work and just went back into the loop each time until I plugged my charger in first.

And no my battery wasn't low, it had 87% when it finally booted up and was at 98% when it decided to go into a loop.

Hope this helps