Q: i tunes library disappeared

What exactly do you mean?  iTunes has several parts and any one of them having problems can affect the others. 

What are the iTunes library files? - http://support.apple.com/kb/HT1660

More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management

What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.htm

Where do you normally keep your iTunes folder?