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 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)