Missing Music in iTunes library

Hi,

I started up iTunes this evening and was just looking at my library list and there are a heap of recently added songs missing. I opened up a finder window and typed one of the missing songs name in to the search box and it located it in the iTunes folder. I double clicked the mp3 file and it maximised iTunes and started playing but I could not locate it in the library. I typed the songs name in the search box in the top right and the song, according to iTunes, was not there despite the track still playing. As I said this song was previously showing in iTunes and is not now. Can someone help me find out what is happening?

Thanks