Check that there is power going to the system. Along with making sure that the electrical cord is plugged in and checking all breakers and fuses, you can use a voltmeter to make sure that power is actually getting to the unit. A voltmeter connects directly to the electrical system and provides an accurate reading as to how many volts are running through the device or outlet.
Check to make sure that the oil tank has sufficient fuel.
Check to make sure that the unit is turned on and that the emergency switch has not been turned off.
Push the red reset button once, holding it down for one second. This is a small button generally located on the top right side of the unit. Make sure that you do not push the reset button more than once.
Check the fuel filter. If you have not replaced the filter for some time, it may be clogged, in which case you will need to replace it.