Hold the power button longer, for about 3-4 seconds or hold volume down pressed and than press the power button. It should power on in bootloader mode, simply press power again and it should boot into system now.
Posted via nexus7 lte
Nexus 4 16GB ~ Moto G 8GB ~ Nexus 7 16GB ~ Nexus 7 II LTE
HTC Desire ~ HTC Evo 3D - sold