Request you to follow the below troubleshooting steps from the document fix :-
Step 1: Remove external devices
If the Chromebook does not start, remove any USB devices, and remove memory cards from the card reader slot. Disconnect all non-essential devices including external hard drives and other similar devices.
After you have removed all external devices, turn on the Chromebook.
If the computer starts, reconnect devices one at a time and restart the Chromebook each time you connect another device to figure out which device is causing the problem. You are done.
If the unit still does not start or exhibits the same problem, go to the next step to perform a hard reset.
Step 2: Perform a hard reset
A hard or forced reset erases all information in the computer memory. This forces the system to clear and reestablish the software connections between the BIOS and the hardware.
A hard reset does not remove any local files or settings.
Press the power button and refresh button for five seconds. The computer turns off, and then turns on again.
If necessary, press power button again to turn on the laptop.
If the issue persists, continue to the next step.
Step 3: Perform a system recovery
A system recovery completely erases any local files or settings and returns the computer to its original factory-shipped state.
If a hard reset does not resolve your issue OR your Chromebook shows the error message Chrome OS is missing or damaged, you might need to perform a system recovery. For detailed instructions go to Chromebook Help Center and search for Recover your Chromebook.
Please find the document HP Chromebooks - Chromebook Operating System Does Not Load (Chrome OS) link
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