I've had this same problem, too - it happened after I installed the patch that came out after Late Night. I've done a complete uninstall/reinstall and here's the things that have happened.
1. I reinstall vanilla, WA, and HELS, but don't patch after I finish installation. The game works, but all the sims have black skin and neon hair, and the game freezes when I try to save.
2. I reinstall vanilla, WA, and HELS, but don't patch after I finish installation. The game insists it cannot run because the base game is not compatible with the currently installed expansion pack, but it also insists that the game is up to date.
3. I reinstall the games, and I patch after installation. The game cannot run because the base game is not compatible with the currently installed expansion pack, but the launcher claims the game is up to date.
4. I reinstall only vanilla, and regardless if I patch after installing or not, the game says something along the lines of "The Sims 3 is not up to date with the latest patches. Run anyway?". If I run it anyway, the game freezes within minutes. If I don't run it, the next time I try to open the game it won't work.
It didn't occur to me to run a defrag, so I'll try that next, but I'm almost afraid that the only fix might be to replace the base game or buy Late Night. If that's the case, I'll probably just walk away. (It'd be the Spore controversy all over again...)
EDIT: Tried a defrag and disc clean this time before reinstalling. Worked.