How to look up lost iTunes shared library password?
Your Home-sharing password is your Apple ID password. It should exist in your keychain:


Type Keychain Access in Spotlight. It should appear in the drop down menu. Open it. 
Select your Login Keychain 
Type in iTunes Store Password in the search bar of Keychain access. 
Double click it and tick "Show Password"


If this appears: 



Type in your user account password and click allow. You should then see your Apple ID password in plaintext.