Play/Pause button stops working with unknown cause

Found this answer on reddit, and it worked:

From gsdgrad05 in Apple Community:

I found a potential solution to this issue. I signed out of the Apple Music application and re-signed in, and now all media keys work across the top of the Magic Keyboard!

1. Open Music app
2. Go to Account
3. Click Sign Out (Media keys should work now)
4. Under the Account dropdown, click Sign In.
5. Type Username, click Sign In
6. Type your Password for iTunes Store/Apple Music account and click Sign In

Maybe it helps others too. And hopefully Apple finds a way to prevent that from happening in the future.