Thank you for the replies.  I haven't noticed anything else wonky with this computer.  I'm using Avast for antivirus, but nothing's changed with it recently.

Maybe it's just a fluke, then? If you're correct, Pescado, I am not sure what would cause that.

I guess the lesson is if I am going to spend hours and hours setting up a neighborhood I need to be way more obsessive about backups.

So, I made a brand shiny new neighborhood with all the subhoods (I have every EP except AL) using the clean templates.  I spent hours lovingly crafting townies for it in CAS and used the Inge teleporter to put them in the game as townies.

Then I loaded another neighborhood so that I could package up an unoccupied dorm that I wanted to move to the university in my new neighborhood.

When I re-loaded the game, my brand shiny new neighborhood didn't appear in the menu! It was gone.

So I loaded it in SimPE.  All the sims are gone and there's just a bunch of "UNK" stuff in there.

What on earth? I am assuming there's no way to get this neighborhood back in working order, but does anyone have any idea how this happened so I can make it not happen again?