There was a freeze warning in my area over a year ago. We had the Heating component checked the day before the warning. The worker declared there was no issue with the oil furnace. The next day, I had dinner like usual. The whole house felt really cold though. Wondering why, I looked at the thermostat, which had been set at the normal temperature. The screen on the thermostat was totally black! No temperature readings, no working sounds when I tried to raise or lower the temperature. So next I tried cleaning the crud inside the heater. I thought that might be the reason for the thermostat not working. Then I looked at the ductwork associated with the heater. I then scrubbed my ductwork, changed the furnace filter and cleaned everything near the heating system. I tried multiple youtube suggestions but I got nowhere. Meanwhile, the lake house got more and more cold. I tried calling the business who’d been there the night before, and many other Heating plus A/C maintenance companies. All of them were on other appointments and could not get me in for the day. I then realized what the issue was. I simply needed a new battery in my thermostat. That was the reason it would not connect to my furnace. I want so worried about the heater, that I did not think the thermostat was the cause. A pack of double A batteries fixed all my issues. The heating component ran great and the big freeze did not affect me.