Up until last week I have had hardly any issues with my Sims 3 game, and it has only been until I decided to install THE SIMS 3: SEASONS expansion pack that I have had issues. My week, summarised:
- Installed SEASONS, game crashes every 5 minutes. Looked all over the internet for a fix/word from EA to no avail. Gave up and uninstalled.
- The majority of my game content was not showing up in game. Most of these 'missing games' had not been showing up in the launcher since installation, but were still present in game. Now, my game was absent of all HIGH END LOFT STUFF, AMBITIONS, FAST LANE STUFF, LATE NIGHT, OUTDOOR LIVING STUFF, GENERATIONS, and TOWN LIFE STUFF, content. It did, however, show SIMS 3 (BASE GAME), WORLD ADVENTURES, PETS, MASTER SUITE STUFF, SHOWTIME, and DIESEL STUFF content.
- Again, I searched the internet and help forums for answers. And again, I was left once more to uninstall the entirety of my Sims 3 collection.
- Re-installed each game in the order of release.
- Launched game, found out once more that these same games (HIGH END LOFT STUFF, AMBITIONS, FAST LANE STUFF, LATE NIGHT, OUTDOOR LIVING STUFF, GENERATIONS, and TOWN LIFE STUFF) were not present within my game, as well as the WORLD ADVENTURES expansion pack.

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I have emailed EA support about this issue, posted in their forums, and waited for hours on end for somebody to assist me with their 'live chat' function - and, of course, no reply

That ^ is what I have written/what is going on and what steps I have taken so far to fix these issues. I am getting absolutely desperate now as I have paid for all of these expansions, and now I am only able to play with the content from about four. Bah.

I'm sure you don't want to hear this, but it sounds like the EPs and SPs got corrupted somehow, and then weren't properly uninstalled, meaning that the reinstall failed to fix them. You might want to try installing one of the old EP patches for an early EP such as WA, just to see if that improves things. If it doesn't, I'm afraid the best option is probably a clean uninstall as per Game Help:TS3 Uninstall^wiki - that should get everything back to a clean slate, so that when you reinstall, everything works. Just in case, when you're installing, check the launcher after every EP/SP install so that if anything does get deleted, you catch it early and don't end up wasting your time installing more EPs over the top of a broken install.

Yep, I've just uninstalled everything all over again and am reinstalling then launching to make sure it's in there. Only up to ambitions, this will take a while... I'll report back when I'm done.
Thank you so much whiterider for your speedy reply!

Don't forget deleting all the extraneous crap that gets left over after a regular uninstall.

Had the same problem and I did what whiterider said and my game worked fine!

Yep, all works! Took ages (although I did spread it over two days), but at least now I have my Sims games back and running smoothly. Still no word from EA though!

Thank you, thank you, thank you again! :D