I have a modine PA50AB hanging gas gargae heater. Last night while it was running there was a pop and blue flash on the back side of the heater followed by a burnt electrical smell. I removed the covers and nothing appeared burnt and nothing looked abnormal. The pilot light will stay on but the heater will not turn on anymore. The gas is on and the pilot switch is in the on position, venting is straight up through the roof. Any help would be appreciated.

If there was a blue flash and burnt smell, you obviously have a burned or shorted wire somewhere. Could have broken inside the wire sheath and you may not see much but the smell would indicate a burned wire. Those are pretty basic units, thermostat, transformer, fan, maybe a fan relay switch in there too.