I've slowly reinstalled programs and periodically verified that the DVD drive sees files, and it appears that the culprit was/is VLC video player. I recently installed VLC (latest version) and now selecting my DVD drive in File Explorer again lists no files and simply shows the message "Drag files to this folder to add them to the disc." I tried uninstalling VLC and rebooting, but it seems the damage (whatever it is) is done. I suppose my next step is to again reinstall Win 10 and start over(!); fortunately, I'm retired and have the time to waste.