If the "Wii U" screen is just on the Gamepad, hold down the power button on the Gamepad and then use the power button on the actual Wii U console to turn it on. If it's on your TV screen, hold down the power button on the Wii U and try to turn it back on.
If these methods don't work, you can contact Nintendo at http://www.nintendo.com/consumer/chat/en_na/chatStart.jsp?source=vru or by calling 1-800-255-3700.