The first thing you should do if your humidifier stops working is to check the water level. When the water tank gets too low, or empty there is an automatic shut-off function in most humidifiers that will stop it from working. Simply fill it back up with some filtered water.
Replacement parts for your humidifier should be changed often, such as the filter. If these parts become clogged it will not only reduce your humidifiers efficiency, but also possibly damage the unit itself.