Media Center's Library is a database of files on disk. If you use another application (such as Windows Explorer or the Finder) to move or delete files that have been imported into Media Center, then MC will no longer know where they are, and the files will become broken links. All metadata is preserved, but the files cannot be used until they are reconnected to the source media files on disk. These files are shown in MC with a red X over their thumbnail or image.
To reconnect them, you need to modify their [Filename] Library Field so that it points to the files again properly.
Media Center can automatically clean up broken links in your Library with the Fix Broken Links function of Auto-Import. However, the following smartlist can be used to display all broken files if you need to work on them manually: