iPhone - Music synced from iTunes on Windows (for over a decade) now "...not available in your country or region"

Since upgrading to iOS 26 my music synced from iTunes on Windows has vanished from my iPhone

When I connect my phone to my PC via USB, under "On my device" it still lists all the synced music, but it's nowhere to be found on my iPhone in Apple Music. There doesn't seem to be an iTunes app for the iPhone any more.

Where is the music and how do I play it?

Curiously, all my playlists are still there, but none of the actual music tracks.

After further experimentation in the playlists, I find that ALL my music tracks are now "This item is not available in your country or region".

What the f&^%&? These are all tracks uploaded from CDs I purchased ages ago.

Thanks Apple