garage band instrument library missing

I'm getting an error message that says my instrument library cannot be found. When I go to support page, it says that I need to reinstall garageband. I go to the app store to download it, and I can't because it is already installed, and it says to go ask for updates from the apple menu. There was an iLife update there, but that didn't fix the problem. How do I get my instrument library back to where I can use GarageBand?