Apple Logic Pro: MIDI Track Gone "Grayed Out"

I was working on a project last night and I muted all MIDI tracks to listen to the audio recorded track.

Then I unplugged a MIDI interface and restarted the Logic.

I was surprised to see all of my MIDI tracks went to "Grayed Out"

It turns out what Logic did was to muted all of the MIDI regions when it found were unassigned MIDI track. Reassigning the MIDI channel does not do anything.

Instead you need to go to the Region menu and un-mute all of the regions.