An update from EA on the missing characters (posted 10/21 on EA forums at this link):
The team are scanning the complete database for that issue and run the fix on every account where she is stuck. It can take a few days until all the games are fixed. Many, many people are playing this game after all.
For other characters (not Lisa or Homer) you can put the building the character came with into your inventory and back into the town again. This will reset the character.
The devs are also implementing a fix with the next build that should avoid that characters get stuck in the future. Until then they’ll run the scan and fix everyone that has the issue.
This was posted before the level 36 update. I’m really really hoping that they fixed it so characters will no longer go missing.
If you still have a character missing and they are attached to a building, try storing away the building and place it back out. If they aren’t (like with Homer, Lisa, Maude, etc.) then the best way is to contact EA through this page.