Well, thank you, WYSIWIG. I went to where you said, and mysteriously enough, "Midi Input Active" was right where it was supposed to be, i.e., where you said to move it to. But while there, I noticed the button "Restore defaults," and I clicked it. That brought the picture of a piano back.

As for the question of what went wrong, I suspect the unwitting action of some third party. I had been downloading medieval music from various web sites, and I have noticed before that when I do that, that Noteworthy or my MIDI settings have been tinkered with. For example, my computer starts playing "Rock of Ages" on the hurdy-gurdy rather than the flute stop it had before.