One of my albums are gone

Earlier today I was wondering where that one song had gone off to. And so I checked out my library without any luck. Then I decided to check out the iTunes Store, I searched for the artist and all albums were there, even the one that was gone from my library. However, normally you'd see the artist name under album name, but it just said 'Unknown'. And I couldn't acess the album. What's happening? 😐