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?