For most Chromebooks, follow the steps below:
Turn off your Chromebook.
Press and hold Refresh button (it looks like a arrow going in a circle). While holding the Refresh button down, tap Power button.
When your Chromebook starts up, you can release Refresh button.
On tablets: Press the Volume Up and Power buttons for at least 10 seconds, then release them.
Here is a video of how to do it: Click Here