There are a couple of methods that can potentially get your music back into place. One would be to remove the existing (missing) entries, then re-download the content from the iTunes Store. The second, and likely easiest, would be to remove the existing library altogether, import its contents to a new library, then download the remaining content from the iTunes Store.
After quitting iTunes, browse to the location of your iTunes Library, and move it to the Desktop. Then follow the following article for steps on creating a new library. Once the new library is created drag the 'iTunes Media' (and 'iTunes Music' folder, if it exists) folder on top of the iTunes window (that should now be displaying a new, blank library). Once the import process is complete, remove the iTunes Library folder from the Desktop, and download remaining content from the iTunes Store.