Location of Itune Music

This means that the location Music used to have for this track is no longer there.

Was the music located on a shared or removable drive that is no longer there?

Under Music -> Preferences -> Advanced it will show you where the media folder is located. If that location no longer has your music, you can try and do a search for one particular track and see if you can find it on the MAC and either copy it back to where the media folder is, or if all the music changed, change the media folder location to where the track is.

Example: The song Come Together is missing.

Hit Command + Spacebar and type Come Together. This will search the Mac for the file.

Where was the music stored before? Is that location still there? If there, is any music in that location?

As a last resort you could try and use Time Machine to go back to a previous time it may have been working, but there's still some unknowns as what happened to the previous music.