'Cannot load game because of uninstalled expansion pack'??

[Ally3772]

I went to continue my game because I am doing a story in the Misc. section called "Starting From Scratch" and I saw the game icon was kind of faded. At the end of the list it had a second thing that said something a long the lines of 'Cannot load game because of uninstalled expansion pack'. I haven't uninstalled anything, so why is it saying this? And is there a way to get my saved game back? Please help me because I am kind of far into my story (Farther then the posts on the board) and I don't want to start over!