OK, hopefully this helps you guys, this just happened to me.
Highlight the track or data that is highlighted and go to the menu - Edit, Unmute. You can also press Shift-U.
Crazy but Edit, Mute (Shift-M) mutes the track but doesnt make the M toggle button yellow, it just mutes the midi notes I think.
I “muted” the notes by rendering in place with the render setting - Mute Source Track selected. Right after it stopped playing so I knew it was related.