Hi and welcome.
We are at a great disadvantage here. Put yourself in our shoes.
We don't know what Mac you have (Model, Year) nor what Operating System (OS) it is running. For that matter which version of iTunes you are presently using.
Do you have iCloud Music Library checked in preferences?
Do you subscribe to Apple Music? If so, has your membership timed out? This could explain a lot.
I know it's tedious, but without that information, we are in the dark and we do very much want to help.
Also, has anything "happened", been installed, updated between the time when all was well and now?
And are you saying that all your music has been erased, or just downloaded music? What about music imported from your own CDs?
As regards retrieval, that depends on answers to the above and - crucially - whether you have a Time Machine or Cloned Backup.
Please post back. We really want to help you.
Ian