Same thing happened to me, iTunes support was pretty much no help at all, chances are all your music is still on your hard drive in your iTunes music folder which is found in my documents, you will have to start the iTunes program, then click on "file" and then "add file to library" or "add folder to library" and send everything back into your library.
I can't tell you why the music disappeared, iTunes support couldn't tell me either, it took three emails (the third being rather rude, before I got a response other than an apology for my download being interupted, which had nothing to do with the problem I described to them, I went from 5600 songs in my library to 1400, now they are all back in there but I pretty much had to figure it out on my own, hope this helps.

Glad to no its not just me Paz. Thx for taking the time to reply, will have a look.

Yeh i am using iTunes 6 Cameron, never had any problems but recently it happens all the time, songs end up with an explanation mark next to them and saying that iTunes cant find em'?