Currently Being ModeratedJan 20, 2013 9:49 AM (in response to ajay67)
Yes, it is a long-standing display bug in iTunes, that has been reported on both the Windows and Mac versions for several years.
When it occurs, the tracks really are in your library, but do not display in Music view. Usually they do show up in Recently Added, and of course, the files really are in the folders where they are supposed to be. Does that match what you are seeing?
Home remedies that people have reported may fix it are to restart your machine, or to download a song from the Store (e.g. a free single).