Cardbook support is offered by the author here https://gitlab.com/CardBook/CardBook/-/issues or the download page shows an email address you can communicate with directly. https://addons.thunderbird.net/en-us/thunderbird/addon/cardbook/

I eventually solved it, but can't explain why it worked:

1. I restored the Thunderbird folder from an archive copy from a few days ago. When I opened Thunderbird the Cardbook view was correct.
2. I closed Thunderbird then re-opened it and Cardbook was empty.
3. I repeated this several times and the fault was consistent. I therefore concluded that Thunderbird was over-writing on of the restored files with corrupt data. I tried to locate which file it was by selectively restoring files and/or folders
4. None of the folders in the root directory made any difference so I tried restoring just the first half of the sub-folders when listed in alphabetical order. This succeeded in restoring Cardbook. I assumed I would be able to close in on the problem folder by selectively restoring folders within the subset that fixed the problem. However, before doing that I closed Thunderbird and re-opened it to confirm that the fault still existed.
5. Much to my surprise the fault was fixed! I can't explain why restoring just a subset of the folders fixed it when restoring all of them didn't.