OK, I know I should just suffer in silence and keep this to myself, but now that it's over....
It didn't get down to freezing here in the Arkansas River Valley last night--all the way up to 33 degrees. I had just filled the empty propane tank the day before. Around 2:30 this morning, the furnace quit coming on. I drug myself out of the comforter to see if I could do anything to get the heat back on. Flashlight--refused to come on for the first time: good batteries, just bad contact. NO LIGHT. Turned on the big outside floodlights, but could not find the switch to the utility lights in either compartment which I needed to get an extension light I had (AC power). Finally got that light. Could NOT find anything I could do. Connected a small electric heater and shut the wonderful sliding doors into the bedroom where we made out the cold night.
Believe me, all this was much worse to experience than to tell.
This morning I have made extensive diagnosis of the problem which involved moving the propane tanks several time. The first thing I found is that the recently filled tank was still full. Thought the change-over valve must be defective. Enough boring details. I now believe that the fault was that the full tank was not correctly connected to the propane line (It was on the 'street' side -- left as facing forward). Guess whose fault that must have been. I have learned a valuable lesson the hard way.
Be very careful in connecting the propane tank after filling. Be sure the threads are properly aligned and tightened.