Q: missing music

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

Yes, I called to support with same issue and resolved! 

and the second link of Limnos saying,  I did that and worked.

Orestonian,

I have the exact same issue and have a thread going under this community forum.  Spent 1.5 hours with Apple iTunes support with no resolution,  My library is stored on a Synology NAS and has been for 3+ years through several OS and iTunes upgrades with no issues.  However, with the 12.3.3.17 my entire library is pointing to the iCloud and playing from there as well meaning I'm using my Internet connection.

I still have my entire library located on my Synology NAS but it doesn't see it.  I have created a new Library but again it only opened the iCould content and nothing local.

Have you tried the advice here? Empty/corrupt iTunes library after upgrade/crash

tt2