My phone is doing the same thing. It shut down about 4 hours ago and refuses to come back on. I've charged it (battery was in the yellow when it went down, but was on the charger), tried taking the battery out and leaving it for a few minutes, pressed the darn power button a million and three times and it just refuses to come back on. Although it did flicker the HTC screen a couple times it turned right back off. It's shut down randomly before but it's always come back up decently soon. This is the longest it's ever refused to come back on.
if you have it plugged into a charger does the LED light on the top left light up and if it does what color is it?
I've had my phone for over a year now and just had this happen for the first time. The battery that was in it was fully charged when it happened. I tried taking the battery out and turning it on, and that didn't work. Neither did plugging it in and turning it on. Switching batteries did work. However the battery I switched it to was my old one that had mostly quit charging. So I'm hoping that after It charges for a bit, I can put the other one back in and get it to start.