O.K. it finally happened to me. My sims started going down invisible stairs in the floor. I was so upset. You see I'm in the process of filming. Lets just say that could really ruin a scene. Any way I searched the web for answers on how to fix it. It was not looking good. Aparently this is side affect to using the "moveObjects on " cheat code. I guess it happens if you use on a particular lot over and over.  All the blogs said that there is no way to fix it except move out. I was not moving out, I just got the set just the way I wanted it. So I decided to try something. I went to Edit Town and evicted my family. I had them leave everything in the house. Then I just moved them right back in. All their stuff was still there  and what do you know, they started using the actual stairs. What a relief. So if this happens to you, try this. It works.