The on/off on some models takes a feed from the mains, unplug it take the side off and check if the cable has been pulled to tight and pulled this off, It only takes a light pull on the mains cable to cause this.
It should be easy enough to check the switch with a meter or a battery and bulb. You could also do it with a test bulb and a couple of spade connectors.
Big rocker switches like that come in standard sizes and you should be able to get one from Farnell, RS etc or the maker.