Sim dissappearing in secret lab (strangerville)

The sim is actually still there. I can click the button to center on him and it goes to where he is supposed to be or I can also have him walk back out of the basement and he reappears. The problem seems to be that nothing shows up in the basement, no sims or equipment.

Yes, I tried that and it didn't work. I also tried using another sim and the same issue occurred.

I tried that but it didn't work. Here's a better way to describe my issue. He becomes invisible once he goes into any room in the basement other than the one with the stairs. He is still there, but I can't see him. I can tell him to walk back to the stair room and he reappears. It's not just the sim though, there is nothing in the basement except for empty rooms. The first screenshot I centered on him after he walked into a basement room and the second screenshot just shows how the basement is empty.

How did the sim enter the basement?  A few months ago I did an experiment with a vampire sim using the bat ability to go into the basement even though the key card door was still locked.  The idea was to get as much of the analysis and vaccine making done as possible before unleashing the spores.  Sim made in just fine, but once out of the hallway the situation was as described in the problem.  Evidently, the downstairs rooms are unlocked for viewing and activity only by that door being unlocked by the keycard.

I'm having the same issue. its been this way for a while now and even in a new save and new game after going through my mods. When i enter the secret lab (even the top floor) the sim nor any furniture show up (ie the files and desks etc). 

I was having the same problem with my sim disappearing in the secret lab and the furniture inside not appearing. I finally figured out that it was either a mod or a cheat that was conflicting. I had TwistedMexican's Secret Lab Build Buy mod installed and the always free build cheat turned on. I turned off the cheat and uninstalled the mod at the same time so I'm not sure which was causing the error but if you're sure a mod isn't conflicting, check to make sure free build is off.