Purchased music no longer available in my library

Make sure you are logged in with the same Apple ID that the music was purchased with. If the music was inadvertently hidden you can unhide it in the iTunes Store: Hide and unhide music, movies, TV shows, audiobooks and books – Apple Support (AU)

Yeah but why after I’ve downloaded and been listening to it for months does it suddenly disappear. That is utter BS if you ask me

In particular why are some songs that I downloaded I have a subscription to Apple Music suddenly no longer available in my country or region? For example I can feel it by sickick is no longer available I have it in my library but can’t listen to it

The distributors/publishers decide what's available.

I really don't know why. But a track from an album I have been listening to has become "not available", and is only available by purchasing the album.

Yeah but because I paid for the years subscription to Apple Music and can download as much music as I want I can’t actually pay for the song or album it’s not an option

I wasn't suggesting you purchase the album/song, it was a just a passing comment. I don't make the rules but not all music is available for purchase or streaming, that's just the way it is.