There are a few things that might have gone wrong:
You might be looking for something that has been either moved, or simply no longer remains posted.
Recently (December 2007) There was a large amount of reorganisation on this site - feel free to have a look around and see if what you're looking for is still posted - or alternatly, you can contact the webmaster and ask about the particular peice in question.There should be a response within two days, most likely sooner.