Before you go too far take out the wires from the solenoid plug and check if you get a change in the voltage when you hit the switch. If you do not then it's electrical, if you do the it's pneumatic.

If it's electrical, do as Jonny says and go through the system. It may be that a wire is loose in the relay or something.

If it's mechanical, check you have flow at the valve (take pipe off turn on bottle) If you do then check the pressure it's running at with a pressure gauge. if that doesn't work, disconnect the outputs from the valve and see if it fires.