From https://support.google.com/chromebook/answer/1080595

When your Chromebook’s operating system (OS) isn’t working properly, you can recover it. Recovery is removing and reinstalling the OS.

You might want to recover if:

• You see this error message: “Chrome OS is missing or damaged”
• You’ve tried other troubleshooting steps and nothing has fixed your issue
• You’ve asked the experts in our help forum for help fixing your issue, and they recommend recovery

See the article for more information and detailed steps.