Fear my catalog is corrupt. Previews have disappeared. Have tried deleting previews.lrdata

Something strange has happened to my LR Classic Catalog and I can't seem to fix it.

The photo previews & thumbnails are not displaying unless I manually click on each file & allow it to load.

I therefore thought it was a preview error & deleted the *previews.lrdata file, hoping it would rebuilt itself but it doesn't.

After I have deleted & reinstalled Lightroom.

I have tried exporting folders as separate catalogs with a mind to reassimilating them into a brand new Catalog.  The exports succeed but each warns that x number of photo previews are missing.  The individual catalogs will open but with the same problem remaining, and give an unknown error if I try to import them to a new catalog.

My backup catalog seems the same.

I fear my catalog is corrupted.  The content of the library & edit information seems intact.  Can anyone advise what's wrong and possible solutions to repair or extract the edit information please.

Many many thanks.