One way to identify the broken links in your library is to create this set of playlists:
Lost & Found playlists
Create a playlist called Found, select everything in Music and drag it into the Found playlist. Create a smart playlist called Lost matching All the rules Playlist is Music and Playlist is not Found. Your lost tracks will be in this playlist. They can be deleted with CTRL+A then SHIFT+DELETE.
Or use iTunes Folder Watch and enable its option to check for dead tracks on startup.
If you have any Apple devices that could hold the lost media see this user tip.
Finally, while I'm handing out advice, see this backup tip to avoid this problem again.
tt2