I had a peculiar version of this that I'll document for posterity.
Phone appeared to not be switching on - when charger plugged in - no light appeared.
However, when the power button is held down the home and back light buttons flash approx 12 times.
The surprising solution - which I found here [http://forum.xda-developers.com/showthread.php?t=2284938|HTC One won't turn on, flashing Home and Back buttons] was to shine a very very bright light onto the phone. I was then able to unlock phone and fix the issue.
(I had set Nova Launcher to enable night mode during a certain time window)
Hope it helps someone someday
(HTC1 M8 Custom ROM)