which file? the only real way is to trash the items "Previous iTunes Libraries", "iTunes Library", and "iTunes Music Library.xml" from within the folder "iTunes". DO NOT DELETE the folder named "iTunes Music" because that is where all your songs are. then launch itunes again, ignore its complaints about lacking a library, and then drag the folder "iTunes Music" into the main window. it will once again add all the songs into the library, but you will have to make your playlists all over again. of course this all goes on the assumption that you told itunes to copy the music into its own folder, and to keep it organized. remember that you can 'backup' your playlists by selecting 'export song list/library' from the file menu.