I've had this problem happen a few times, here's what i did, go in your music folder where itunes stores that specific album, copies the whole album one a external disk erase the original from the itunes folder,make sure you empty you trash, start itunes and try to play one of the song, itunes will tell you it can't find the song press cancel then erase all the songs you have in your itune library of the same album and then re-copie
the album from your external drive to itune library and everything should be ok hope this helps
(it's important that the information in the album info is the same for all the included songs, same album name same artist,etc.)