[Rathji]

There might be an easier way, but this is how I deal with that issue.

Select all the songs that are missing and remove then from the library. Since it is all your songs, then it is easy. Then in file menu, choose add folder to library, and select your itunes folder, or wherever your music is stored. Chances are, since you don't know where the music is, it will be located in your c:\Documents and Settings\<user.name>My Documents\Music\itunes (for windows XP) or C:\User\<username>\Music\itunes (for Vista or Windows 7). It will readd, and get album art etc.

I wish you could just add a song and it would figure out that it was a duplicate of a song already in there, so you didn't have to manually erase songs that somehow got added twice. Maybe 10 will do this, but I have not had the experience to be sure.