Bought some music on Apple Music and now most of my it is gone

You bought it from the iTunes Store, when was this? If it’s been awhile , it may no longer be available.

First check they are not hidden - this needs a computer

Hide and unhide Music ...

https://support.apple.com/en-ca/HT208167

Or ....

Go to Settings > [your name] > iTunes & App Store.

Tap  your Apple ID at the top and sign out 

Close Settings.

Go back to Settings and sign back in 

Go to the iTunes Store to find the music, tap the cloud icon to download 

Or do it this way:

On an iPhone, iPad, or iPod touch

1. Open the iTunes Store app.
2. On your iPhone or iPod touch, tap  More in the menu at the bottom of the screen, then tap  Purchased. On an iPad, just tap  Purchased. 
3. Depending on what you want to redownload, tap Music, Movies, or TV Shows. If you use Family Sharing, tap My Purchases or choose a family member’s name to see content that they purchased.
4. Find the movie, TV show, or music that you want to redownload, then tap it. 
5. Tap  to the right of the title. The movie, TV show, or music downloads to your device. You can find movies and TV shows that you redownload in the TV app§ or Videos app, and music in the Music app. Don't see the item that you purchased?

This link should be of further help: https://support.apple.com/en-ca/HT201272