I"m assuming this isnt a physical copy?
If it's digital, you probably got it from Origin, which is.... the absolute single worst launcher I've ever seen, and one of the worst PROGRAMS I've ever seen. It's THAT bad. Expect things to not be... easy.
One huge tip with Origin: Back up your games like CRAZY. Over and over and over and over and over. Do it, or you might regret it. Do it FREQUENTLY.
In any case, you have to redownload the expansion from Origin itself. Yes, I mean redownload, not re-install. Origin is SO bad that it doesnt save installer files correctly. ALWAYS just redownload the entire thing from scratch.
It IS possible that you may have to uninstall, download, and then reinstall the base game as well. Theoretically, you should still be able to (maybe) pull your actual save file out, copy/paste it to somewhere else on your drive, and then stick it back in after the redo is finished. I cant guarantee this will actually work, but it might. It's worth a try.
If you DIDNT get it from Origin, let me know where you did get it from, and I might be able to help, though I cant guarantee it.