Hope This Helps:
"I think you encountered the issue with a stuck power button. I had the exact same behavior that also went away without explanation to return a few weeks later. I read that one trick is to bang it in a hard surface and shock the button out of its stuck state.
The second time it occurred I actually found a much more gentle way to temporarily fix this. Take an electric tooth brush without brush and push the oscillating shaft onto the backside and side of the phone close to the power button. After a few seconds of that my phone was back in working order".