I've been playing on a save and I have had the stairs for some time (I'm playing the Legacy Challenge). For some reason, all of a sudden, sims cannot go up and down the stairs. It is not a CC stairway and it's base game (I think). There is no railing. I've attached pictures of what the stairway looks like and what is around it. I've searched online, and it says that there may be a hidden item or something blocking the way. I've tried deleting and readding the staircase, using a different staircase, and exiting and entering the game again. However, nothing has worked. It's only adults in the household. How can I fix this? Can there be a way to find if there is an item blocking the path? I've tried turning buydebug on and I don't see anything. Any help is appreciated. Thanks!

Have you tried going to edit town, moving out the household (with their house), bulldozing the old lot, then putting the household back on the same lot? Nraas' Master Controller can be used to list all the items on the household lot so it might be able to list off something that is on the lot that could be the block.

Also check for mod conflict with Sims 3 Dashboard

Sounds like a jig is there.

MC Objects Stats can be used to flush those, but deleting the lot and replacing it will also do the trick.