@VampyMaria I've split your comment off and moved it to the Sims 3 (Windows) section because the other thread was a couple of years old and pertained to a (now long gone) mac-specific issue.
Please clear Origin's cache and try again. If that doesn't help, you can uninstall and reinstall Origin only ("the hard way"), which wouldn't affect your installed games.
If these steps don't work, you can try to force an automatic download of your pack(s). Make sure that automatic game updates is turned off in Application Settings > Application, and quit Origin. Then sign in again, enable auto game updates, then go to your game library, right-click on the Sims 3 icon, and select Repair Game. When it's done, exit Origin, and when you sign in again, Origin will hopefully start automatically downloading your pack(s). You can manually pause or cancel any download you'd like to skip for now.