I replaced my thermostat a few weeks ago because the heat would not come back on after shutting off when hitting its target temp. Since that time it seemed to have addressed teh issue, but last night it once again did not come back on after turning off when hitting the target temp and we awoke to 60 degrees in the house. It happened again this afternoon when we got home. The furnace will kick on if it is turned off then back on. What might be some causes of this?