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.

Power/running blue lights may be on and flashing. But the usual screen fails to come up.
UPDATED 20210414 with Acer C720.

OMG, it's a paperweight.
Or maybe not.

See https://www.howtogeek.com/190645/how-to-factory-reset-a-chromebook-even-if-it-wont-boot/

1. Press Esc and Refresh and touch Power button. [20210414: smi needed to hold the 3 buttons down.]
2. See Recovery screen asking you to insert the Chromebook Recovery USB Stick (see URL above).
3. Insert USB Stick
4. System verifies USB.
5. System recovers. It takes a couple of minutes.
6. You're asked to remove the USB stick; do so. System reboots.
7. You need to connect to Wifi and login using gmail account and password.

If anything goes wrong in a Chromebook Linux dual-boot or substitution, that Magic Reset Key is how you get back to plain-vanilla Chromebook. Then you put the Chromebook into Developer Mode if it isn't there already, and run the ROM substitution scripts or set up Crouton or Crostini.

The more advanced techniques assume you have already tried and failed to do a normal Reset of the Chromebook.

If your Chromebook is no longer getting Chrome OS updates, it's well worth the effort (and slight risk) of opening the Chromebook, removing the Write-Protect Screw, and flashing a substitute ROM like JohnLewis or SeaBIOS (CHRX) (Reyhout). All of those substitutions are done using the curl command from a Chrome Root Terminal. Just a few keystrokes will get you into the Root Terminal, provided you know your Chromebook's Root Password. (If OEM default, this is published a lot of places.) The rest is just running the scripts and making your selections.

Then the Chromebook will never again try to reset itself, and you will have a pure Linux Chromebook. Not all models qualify.