This is NOT about missing photos. All photos dated after March 31, 2020 disappeared from their catalog. It was not a case of the preview being there and the high res image file being missing. The catalog could not find any photos after March 31. Searching on the hard drive in the right place did show the high resolution photos were within the catalog's file structure.
The next time I opened Lightroom, lo and behold the photos were there. Any thoughts? My catalog now has 40,000 photos. Do I need to split the catalog to have a smaller size? Thanks.