Thanks.
{Menu > Library > Find all missing photos.} yielded 60 images, some of them critical to my body of work.
a) How did they get to be missing (baffling) and,
b) How do I reinstate them - re-link them?
This web link describes the "missing" situation better than I can, so read it start to finish-
https://www.lightroomqueen.com/lightroom-photos-missing-fix/
In my experience I have managed to achieve 'Missing' photos by simple 'errors'-
1. Renamed a folder with File Explorer. eg. Renamed {my photos} to {My Photos}. They are not the same! note the capitals.
2. Moved a folder with File Explorer.
3. Had two Catalogs referencing the same photos and used Lightroom to rename them in one catalog-A, now the other catalog-B has them as 'missing'. Catalog-B is still looking for the 'old' filenames.
4. Forgot to plug in an external drive with the original files- these will show as 'Missing'.
5. If you only have 'Smart Previews' available (perhaps an disconnected USB drive) again these will show as 'missing'.