You didn't mention how old this unit is and if it worked properly before today. Symptoms described could be a number of things - LP supply, regulator, sail switch, high limit switch, restricted cold air return, restricted exhaust tube, etc.
Unfortunately to access the sail switch, high limit switch, circuit board, etc., the furnace has to be pulled away from the interior wall and/or removed (first remove outside exhaust port assembly)
You could try resetting it, sometimes there is a reset switch on top, otherwise pull the fuse for about 30 seconds and put it back and then try. Also check for debris, wasp nests, spider webs etc in the exhaust tube.