I had the same problem and do still sometimes with sims going missing if you use this http://www.modthesims2.com/showthread.php?t=272312
choose the baby thats missing and click snap reset and it will come back...its a black box

I also find that if i play with simpe installed it always crashes.  whenever i install a new game i always play with just the hacks i know that are ok, then gradually put themback in when squinge makes an updated version

Thankyou i have her back......whoo, thought i would have to reinstall game as its my legacy family and i wouldnt have gone on without her.. thank a lot....

Place the hack into your downloads folder, go into game, the black box in in hobbies/misc click on it and snap reset... it works

Thanks treppie i have downloaded it and i will let you know if it works.

I have been playing for a while now with apartment, only put a smidge of downloads back and hacks what are ok, but my sim has dissapeared, just a dot, you cant click on her but you can control her, tried everything, appearence, clothes, gussy, i cant cheat her cause  you cant click on herplease help.......

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There is a way to get securom off your computer

 Follow the guidelines below ripped from http://www.tomshardware.com/forum/98241-13-remove-securom-malware-uninstalling-bioshock-demo:

i myself havnt done it yet.

It may have lost its .exe file, this happened with me when my hubby ran a virus scan and because bon voyage is using securon it thought it to be a virus and quarantined it.
Found this out after i installed it again, you dont need to uninstall first just install the exp pack over it.

I would attempt to upgrade your driver for your graphics card as these need updating regularly.
I had this problem and was solved.