OK, I believe that I found my way out of the problem, at least for now. I use File Hamster Plus versioning software and found a saved revision of the database that had been stored about 20 minutes before the error appeared. So I restored that revision and all is working well again.
Still would be nice to know what caused the problem so as to avoid it in the future. But I can't complain now.
File Hamster has now paid for itself many times over here.
Thanks!
Jim