First, understand what is supposed to happen when the thermostat calls for heat: It would be best to have someone outside and someone at the thermostat to check these basic things. If you check your propane like mentioned above, here's the next steps to check.
1. The fan starts. It moves air through two separate blowers. One is inside, from the cold air return through the heat exchanger, to your warm air registers. The other blower runs off the same motor shaft and is for burner combustion air. It draws in outside air through the combustion chamber, and out the exhaust. Both the outside intake and exhaust are on the furnace cover outside.
--First, make sure both of these ports are unobstructed. Also make sure all of your heat registers inside are open and unobstructed. This is because there is a air flow switch that has to make before anything else will happen.
2. When the airflow switch makes, the control board will start the spark ignitor and open the gas valve. You will know when this happens because you should hear the ignitor sparking for about 15 seconds. If there is no flame detected, the control board will shut the gas valve and run the fan for about two minutes to purge the combustion chamber of gas, and then it will retry a couple more times before locking itself out completely. I think you have to cycle 12VDC power to the control board to reset it.
--Do you hear the ignitior trying?
--Do you smell propane?
--Did you hear it light, feel any heat at the burner exhaust port?
3. When the burner lights, the ignitor acts like flame rod. It has to be in the flame correctly, or it won't work. The control board will shut the gas valve and go into purge and lock itself out, or continue to burn until the thermostat satisfies.
I'll tell you that there is no real good way to troubleshoot and service these furnaces. You can do some limited checking with a volt meter if you can read schematics, but you can't see stuff work. They really have to be removed and put on a service bench by a tech to get very serious.
Hope this helps you gather some info, so we might get you working again. Post your results back