Superb! Thank you, Mr. Foobarly! You were absolutely right, and life is swell again.
When I went to Advanced Prefs, I discovered them missing from the menu. Somehow I had removed Advanced Prefs, although Prefs were still there. I hadn't realized until I was following your suggestion. So I accessed Advanced Prefs via F2 and sure enough, things were as you said; fixed it; no problem. (Now I'll hunt around and fix that menu.)
Thanks again.