Q: where is my itunes music?

It may be because your iTunes Library file (the database that tracks where your media files are located) became corrupted.  The iTunes update (which also updates your iTunes Library file as needed) may have made your iTunes library unusable.  When this happens, iTunes opens iTunes with a new library file, which is blank.

However, your actual media files (like your songs) are probably still on your Mac and fine.  The default location for your iTunes Media folder is (unless you changed it)

<startup disk>/Users/<your user home folder>/Music/iTunes/iTunes Media/

Look there to see if you can find your song files (and other iTunes media), organized in sub-folders by media type, artist, and album.  If you find your media files, you can add them to the new iTunes library, or preferably...

If you use the built-in Time Machine feature on your Mac (as you should) to backup your user data to an external drive, you can recover your iTunes Library file (from last backup before the iTunes update).  If your iTunes Media folder still exists with your media files intact, restoring your iTunes Library file from backup should fix the problem.  Here's where the iTunes Library file is located (by default).

<startup disk>/Users/<your user home folder>/Music/iTunes/iTunes Library.itl

You can also restore other files at the same location with "iTunes Library" in the name, if you see any (the .itl file is most critical). 

If your iTunes Media folder (with your actual song files) is missing (or empty), and Time Machine backed it up, also restore your iTunes Media folder.

ASC user turingtest2 help document: Empty/corrupt iTunes library after upgrade/crash - https://discussions.apple.com/docs/DOC-6561

If you don't see your content after updating iTunes  - https://support.apple.com/kb/HT203225