Boot loop after updating HTC One M8 to Android 6.0 Marshmallow
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