I have the C720 and have experienced where my chromebook will not boot, or the wireless is turned off and can not be turned back on. I have also experienced a black screen. If any of this happens, hold the power button down. You can do a restore, which is pretty easy,
Press and hold the Esc key and refresh (third key from the Esc) and press the power.. The Chromebook will boot to Developer Mode. Press Ctrl + D to turn off OS verification. When prompted to turn OS verification off. press enter and the chromebook will restart.
At the OS verification is OFF screen, do not press the spacebar. The recovery process will start automatically after approximately 15 seconds. The restore process will take around 5 minutes. A progress bar appears at the top of the screen.
The chromebook restarts and prompts to re-enable verification. Press the spacebar.
Press Enter to confirm that hou wish to turn OS verification on.