I did some tests with the savegame amywelshcastle provided.
First I have to say, it's definitely neither an installation nor a computer problem. Reinstalling won't make it any better and is just a waste of time.
Test 1
Played vanilla - nothing added, no Store, no nothing, just the add-ons and HELS (don't have the others stuff packs)
Sure thing I found an invisible daughter using the slide after coming back from school so I reloaded the game and followed her to school to see at what point in time she's turning invisible. She came out of the rabbit hole ok, she got on the bus ok and I could see her driving in the bus. When she got off the bus however, she stayed invisible. If you look how the games handles Sims coming in and out of rabbitholes or cars you see that they turn invisible for a very short period and then appear wherever they are supposed to. It seems to me that this turning visible after getting out of a car/rabbithole is borked somehow making them stay invisible forever.
I played on and seen the mon and father would be invisible too (they did not get in any car or rabbithole so this seems to be some kind of chain reaction).
I tried the cheat resetsim * and sure enough they all turned visible again but as soon as they went to bed, all Sims using one got stuck in bed. Trying resetsim again made them float above the bed.
Test 2
Added ErrorTrap, Overwatch, DebugEnabler, MasterController and Decensor. No other CC.
I saved the game while the girl was in school and tested from there.
I got two scriptErrors from ErrorTrap which I will provide to twallan. I'm not a specialist on reading those so he will need to tell me what they mean. Overwatch cleaned about 127 cars out of the hood. I again followed the girl around. No invisibility, no stuck sims. I let the game run for over a day and nothing unusual happened (they were all miserable but I did not take care of them).
Test 3
Removed ErrorTrap. Same result as test 2, so it was another mod that cleared the problem
Test 4
Removed Overwatch. Back to result from test 1. So it's Overwatch that cleans something that causes the whole problem. I used fix invisible Sims on DebugEnabler. I got quite a few messages for outfits that got repaired on several Sims (fix invisible Sims fixes corrupted outfits) but the poor little girl stayed invisible. So I used reset lot on MC. There she was again but soon after the butler would get stuck on the pond and the whole stuck sim in bed restarted. Entering Build Mode cured that for the mother but not for the rest of the sim and she was stuck again as soon as she got back in bed.
So, what I would like to do is passed this savegame on to twallan. Maybe he can find out some more stuff, especially WHAT cleanup in Overwatch seems to cure the problem. Let me know if that is alright with you, amywelshcastle.
@AndreeAndree: What snapdragon said.
