Good advice on changing the filter, but you can open the trap door and see if you have fuel in the combustion chamber. if not, you have a fuel blockage or air in the line. if you have a bunch of fuel in the bottom you will have to empty it. I wrap a rag on a piece of wire and soak it out. Yep it takes time but beats the he** out of blowing pipes off (don't ask).If no fuel, pump maybe not primed, or nozzle plugged or filter blocked or line plugged. If fuel is entering the chamber and not lighting off, check the electrodes and see if theyre clean. Also there is a photoelectric cell that could be dirty (filmed over). It won't light if so. It should be mounted behind the transformer. DO NOT try to light it off if theres more than 1/2 inch of fuel in the combustion chamber.
If your not sure what your doing call a service tech or take out good insurance before you mess with it. Grump
Forgot the most important thing, TURN the power OFF BEFORE you mess with the transformer.