Leave it plugged in to the wall socket for about 15 minutes or so to give it enough power to boot, especially if it was completely depleted, and then try to start it up. If that doesn't work, make sure the USB cable or wall connector hasn't malfunctioned. If that doesn't work, put it down, let it cool for about an hour and then try again. If the problem still persists, make a genius bar appointment with the nearest Apple Store.