One radical solution is to place a normal predefined stair exactly over the bugged one (with the moveobjects on cheat activated) and your Sims should use the new one to reach the room above.
This is an old thread but it might still have some interesting information for you re the good old Maxis stairs glitch if you haven't seen it already:
http://www.modthesims2.com/showthread.php?t=14349Also I never use Maxis stairs anymore after having encountered this problem innumerable times- I now only use Holy Simoly stairs and they seem to work fine.