I can only assume you let the battery deplete. Anyway, let the device charge for about an hour while powered off. Afterwards, press the power button to turn it on. If it won't power on, press both the power and home buttons until I you see the logo. If it still won't power on, make a Genius Bar appointment with the nearest Apple Store. By the way, I'm assuming the power cord is good and the wall socket has power going to it.