I've got an Aprilaire w/ slightly different controls but as @Streblerm suggests I'd suspect the solenoid valve. Mine went out a few years ago. If you've got a Volt/Ohm meter measure if its getting 24VAC, if so it's either plugged on the valve/orifice end or the solenoid coil is bad either open and shorting turn-to-turn. Look around on the web and you may be able to find a generic replacement. OEM will be expensive AF. Put simply, if you've got water to the solenoid and voltage to the solenoid and still no water flow it's the culprit. If you're lucky its mineral deposits in the jet/orifice but probably not.
Was the furnace running at the time?
So the problem may be on the furnace side.
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