Off the top of my head, The only thing I can think of, it that you might have moved them from the original location so the indexed songs are no longer pointing to the correct location. Sometimes even renaming the drive to a different letter is basically like moving them. For example my karaoke songs are on my K: drive but if I decide I'd rather call my K: drive my Z: drive, VDJ won't be able to find them because it will be looking for them on your K: drive (which, of course no longer exists). If this the case, rename your drive back to the original letter or remove all the folders from the Music Database (you can't edit them), and then re-add them, this time pointing to the new drive letter.
-Fabio
