check circuit braker of this VAC unit first.
check all the rest of circuit brakers in the control cabinet.
find "reset" button (or contact) in the front of VAC unit control board. Reset contacts are at the bottom there, close to the edge of PCB.
note: "check braker" means You switch it "off" and "on" instantly. Main power supply must "off" at that moment, of course.
You will get another alarm, most likely. Power transistor module must be replaced maybe, see the VAC unit manual.