Try this... remove the cover on the upper left side of the heater. There are two thermostats, left one is 120v, right one is 12v. There is a reset button(protruding) from the upper part, push this in and see if there is a click. If so the reset has tripped. Pushing on the rubber insert on the cover sometimes does not reset.
If no luck, where the burner tube goes into the chamber is the spark device. You will see a insulated wire going to a electrode. Carefully remove the electrode. Using fine sandpaper, polish both electrodes to remove any coating. Make sure the gap between the electrodes is <1/4 inch. Reassemble and try again. Caution.. when removing the black cover, there is a ridge around the edge that fits into the space where the 'stats are, use a screwdriver to gently get the cover loose around the entire edge....good luck
