Thank you for this suggestion, Xavier:
<Thanks to try to rebuild the categories tree via the menu Products > Categories, then click on "Rebuild".>
I tried it, and the categories tree rebuilt ok, but the missing listings didn't come back.
Eventually in the Display/Content Menus screen I was checking each menu, and several came up with a message that no module was associated... "click here to associate" ... and now all the missing modules are back and the shop is displaying as it should. What a relief!
Thank you!