Electrical problem (kitchen hood vents)?

Listen to me...! If the switch is turned off, you should have full voltage across the switch -- 120v -- on a voltmeter. If the switch is turned on, the voltage should be zero. That is because when the switch is turned ON, the points at which you a measuring the voltage are actually the same point. If the switch is OFF, you are measuring the voltage before and after -- "across" -- the switch. That would give you the 120v reading.

If you do not understand this...then you should not be fooling with the electric in your home!

A 3-phase motor, so it's not a kitchen in a house. How many wires are on the switch? 2 or 6?

If 2, then it just turns on a contactor like lediy99 suggests.

Call and electrician before some one gets hurt or there is a fire.

I believe the kitchen switch goes to a 3-phase contactor and then to the motor, otherwise you would have a 3-pole switch in the kitchen. You will need to look above the ceiling and see if that is the case. If so the coil on the contactor is bad.
Another possibility is the 3-pole breaker may be off or tripped.

There is always power at the switch. The power is sent through the switch to the hood vent when it is turned on. The switch just breaks the circuit when switched off and completes the circuit when switched on. If you have power at the switch but none at the vent when you switch it on the switch may be defective. Use a volt meter to see if you have power going to the hood vent when the switch is turned on. If so then there is a problem with the wiring going to the vent.