It is assumed that you have power going into the switch, so test with a voltmeter to see if you have got power on the load (fan) side of the switch when in the on position.
If you have the problem lies in the fan
If you haven't got power coming out of the switch-then the switch is your problem.
It is not unusual for steam and grease vapour to mess up the contacts inside these switches
GBW
December 2012