Hello GrahamS1958,
Thank you for reaching out in Apple Support Communities. We understand you're unable to log in with an updated Apple ID password for iCloud on your Mac. We'll be happy to help.
Just to be sure, were you able to follow the suggested steps outlined in this helpful article: What to do after you change your Apple ID or password
If so, and these steps do not help, we recommend to restart your Mac in safe mode. This helps to perform a basic check of your startup disk, deletes some system caches and more. Here's an article with more information and the proper steps to take: How to use safe mode on your Mac Once you're in safe mode, please restart your Mac as normal and see if you can sign in with your Apple ID account without any issues.
We look forward to hearing back from you with an update.
Kind regards.