How do I recover my personal music library after joining Apple Music?

Vanished from where?

The Apple Music Subscription service gives you access to the Apple Music catalog. Unless you enabled the Sync Music Library option, it should not even touch your existing library.

If you were using another service such as iTunes Match for instance, and cancelled it, it could cause loss of music on devices that were accessing that music from the iCloud music library.

With Apple Music active and Sync Music library enabled, it should again match and upload your music from a computer to the iCloud Music library and make it available to your other devices.

Anything that was directly synced from the computer to the another device should remain there, untouched.

If you were relying on the iCloud Music library to store your, music without any other backup of your music that would have been a mistake.

For anyone here to be able to asset, you will need to explain in detail what exactly you did, and where you are looking for your music now.

What were you using before?

Did you just start using this computer with Apple Music?

Did you recently upgrade to a macOS version that replaces iTunes with Apple Music?

Where was your music before this?

If you just subscribed to Apple Music, but where using the Apple Music app before this, then your music should still be available in the Library Section of the Apple Music app.

If you recently changed computers, then your music would have needed to be transferred from the old computer to the new one manually.