Aprilaire 56 Humidistat not getting power
Wired in this way, this should have never worked.  W2 is the terminal that the thermostat uses to activate second stage heat. It is not energized by the furnace, but by the thermostat.  

Also, it looks like the humidistat is not wired properly.  The wires from the furnace should be in the R, and C terminals. It looks like they're in the I-, and R instead.


  
  Figure H from manufacturer's installation instructions.