Try pulling the battery out and letting it sit for 2-3 mins then restarting it.
Right play, but that phone doesn't have a removable battery. It does have a recovery procedure.
Try holding down the power button for a good 10 seconds.
After about half that time, the button lights will start blinking, then it will power down and turn back on - once in a while, you'll need to turn it back on separately, give it a minute (60 seconds) first though.
That does the exact same thing as a battery pull (actually, forces all the silicon to a known state) - and won't lose you any data.
I had that happen with my wife's Evo 4G LTE several times in a row - and then a few days later to mine.
I narrowed it down to one of our charger cables going wonky. Replaced that cable and never saw the problem again.
Exact same thing fixed my buddies Evo 4G LTE as well.
If it persists, think about replacing that cable.
If it doesn't happen again, I'd write it off as just some glitch.
PS - the recovery procedure varies by model on phones with fixed batteries - but for an HTC, it's 10 seconds on the power button.
