Applications :: ITunes Original File Info Deleted - Library Empty

In trying to set up shared music among the users of my computer I think I deleted the original file info, even though there was a music folder showing folders for the individual artists in my library. These however now appear to be empty. I have the music listed in iTunes but when I try to open a song I get a window with the following message, "The song "...." could not be used because the original file could not be found. Would you like to locate it?" But no luck locating the files. The guy at the Apple Genius Bar said I could retrieve the music but refused to tell me how because it is supposedly illegal. I don't know why it is illegal because the tunes were purchased. But he did tell me to google the problem and I have come up empty handed.