Thanks. I seem to have an even more basic problem: somehow Lightroom appears to have disappeared from the Applications folder.
For what it matters, I’m guessing that you are right and that I unchecked the Basic Panel.
That's odd - remember it is "Adobe Photoshop Lightroom."
On a MacBook, show the Launchpad (4-finger pinch on the trackpad) and type "Lightroom" in the search bar to find the app... Pin it to the dock when it's open so you can easily find it in the future (right click the dock icon, Options > Keep in Dock).