Rpi3 - Booting and shutting down continually

Help please. After building up a really useful HA system I now have a RPi3 that simply cycles through boot and shutdown. I have about 30 seconds from login prompt to do something before shutdown is initiated. 1) How do I break the cycle to get time to do something to repair HA? 2) What do I do once I’ve stopped it continually rebooting?

Getting Unable to load the panel source: /api/hassio/app/entrypoint.js. out of the blue when trying to access Supervisor, File Editor, etc. from GI.
As others have done I finally pulled the plug on the Rpi3 to do a hard reset but now HA is unable to be reached at all on GI.

What I’ve tried:-
RPi can be pinged OK but isn’t reachable by SSH so I’ve plugged a monitor and keyboard in to try and see what is happening.

The RPi boots up to the hassio login prompt. From this time I have about 30 seconds before it shuts down and reboots
I can access the host system using ‘root’ login.
I then get the ‘Welcome on Home Assistant’ command line
I’ve managed to run “docker images” which lists repositories, etc
/mnt/data/supervisor/homeassistant returns permission denied

The RPi then closes everything down (like a reversal of boot up showing processes stopped one by one, not blank screen) then it reboots and the cycle continues.

Can anyone tell me how to stop the Rpi going through this cycle? And then what to do after?

Googling seems to suggest
Renaming Config file and rebooting - assume blank Config loaded - then rebuilding the new from the old.
Putting something in to secrets.yaml. Or removing secrets.yaml

As you can tell I’m no expert so need guidance using words of one syllable or less!

Thanks for reading this.

Possibly a corrupt SD card.