If they flip the switch and can only hear humming (usually happens with new units and when it get cold):
The motor runs with a rubber diaphragm that needs to be broken in or warmed up. If you hear humming when you flip the switch and nothing happens, try a couple things. 1) There is a grate with a fan in the front. While the unit is OFF, rotate the fan with a pen to get it going, then turn the unit on (obviously, without the pen in the fan and in your hand). You can also try turning the unit on and off several times to try and jump start it. Once it starts, let it run for about two minutes or so to really warm it up.