I can center the camera on my Sim, and then I can access the upstairs. I didn't build anything or tear anything down. I rearranged furniture and added some more items. I moved the book shelf to the bottom of the stairs, and placed a treadmill at the top of the stairs.
My Sim was sleeping when I entered this. She has a morning routine before I send her to the main living area. There, I will test again, maybe my computer needed to reboot.
Thanks for helping me. I really was at a loss of what to do! I wanted to move my 4-person household to this apartment, trying out city living, and having parties at other places.
@enchantress6274 It would be odd for a few new items to bork the game's sense of how floors work. But just as an experiment, try deleting everything inside the apartment, including the stairs. Use "save as" to rename the save, so your original (from before you deleted the items) remains intact, quit, delete the five cache files in Documents\Electronic Arts\The Sims 3, and reload. For reference, these are the cache files:
CASPartCache.package
compositorCache.package
scriptCache.package
simCompositorCache.package
socialCache.package
If this doesn't help, you can get rid of the save where you deleted all the furniture and go back to testing on your now-current version. This time, evict your sims in Edit Town, save the apartment to the in-game bin, bulldoze the lot, place its bin copy, and move your sims back in. They'll keep their jobs and relationships but lose their promised wishes and opportunities. There are ways to work around that, if you don't want that result, but the first thing to do is see whether this fixes the apartment itself.
