Best instructions:

Hard Reset Chromebook

1. While pressing and holding Refresh , press and release Power  .
2. Continue to hold Refresh .
3. Release Refresh  when the Chrome screen appears.

Original instructions from Google Support

Hard reset your Chromebook

For most Chromebooks, follow the steps below:

1. Turn off your Chromebook.
2. Press and hold Refresh  + tap Power .
3. When your Chromebook starts up, release Refresh .

On tablets: Press the Volume Up and Power buttons for at least 10 seconds, then release them.