Find the house wires to the switch (or switches) and check for voltages being there and then the switches being good.
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Sometimes, if the fan isn't used much, the grease / dirt deposits on the motor shaft build up enough to prevent the shaft from turning on start up, it doesn't take much.
try: Remove the filter to expose the fan blades, spin the fan by hand to ensure that the shaft moves freely - if this was the problem the fan should now work, in which case run it on high for a few minutes.