Welcome to Android Central! Leave it on the charger (on a wall outlet) overnight, then try pressing and holding Power for 10-15 seconds. If nothing happens, do it again, since the first time you do it might actually have forced a shutdown (if the phone had actually been frozen on a blank screen).
If nothing happens, the battery or the motherboard might have failed. You'll have to bring it to a repair shop, if it's no longer under warranty.