edit: Rebooting fixed the problem. I'm still wondering what could have caused it in the first place because this happened once before, but I don't remember what I did to fix it then (at that time a reboot did NOT fix the problem).
I cd to the folder containing the files I wish to edit and from the command line type: code . and VScode opens normally except there are no files listed in explorer to be opened, only the message: "You have not yet opened a folder." I then click on the "Open Folder" button, select the folder I wish to open, and get the exact same result: "You have not yet opened a folder."
I've Googled this and searched here on Stackoverflow but came up with no results. VScode has been working find since last December and I haven't added any new extensions. I'm using a MacBook Pro, not Windows or Linux if that makes any difference.
I've also tried to open the folder by specifying it explicitly: code /Users/davidoliver/sites/todo-redux