Sims get stuck

Diablohorns​  The reason that happens occasionally is because the sims were traversing the stairs at the time you saved.  Sims can't be saved on the stairs, so what happens is the game places them on a clear tile close to the stair, including at times, in inaccessible spaces like a roof or under a stair depending on if MOO is on or off.  So, one way to get a sim out of an inaccessible space is to travel.   Another way is to use the cheat to teleport the sim.  However, that cheat will disable trophies on the consoles as testingcheats has to be on (then you shift click on the sim to get the debug and cheat options).    

Hope this helps.