Sometimes you may open a Pro Tools session and your MIDI tracks are gray (inactive) instead of blue (active). Simple fix!
All you need to do is set the MIDI to output to some source, usually a virtual instrument plugin. You can do this from the MIDI Output selector in the Instrument view from the Mix or Edit Window.
Blue is the default color for MIDI in Pro Tools, but you can of course change the color of your clips. So, if your MIDI is grayed out and not making any sound – simply assign a source to it.