Oh no! What happened to my HVAC system? Heat is not working and it’s freezing winter. But before you make your final assumption that your furnace is at fault, check to see whether you have a broken thermostat.
Learning some basic thermostat troubleshooting methods may save your cost of having an HVAC technician make the diagnosis. 
If you find any of these three warning signs below, there’s a strong possibility that you have some with your thermostat and need to repair or replacement.

1. Your Thermostat is Unresponsive

This could be your first scenario, and because of the same your furnace isn’t functioning and you see a totally black or blank screen on your thermostat, a lack of power may be the issue. Try replacing the batteries, then check for a blown fuse or tripped breaker in the electrical panel. If the thermostat is still unresponsive, make sure the power is off and remove the cover. If it looks dirty inside, use air or a soft brush to clean away grime that may be affecting its functionality. Then go for loose wiring or terminal screws and tighten them up. 
If your thermostat is still unresponsive, it could be a trouble and you’ll have to get an HVAC professional to inspect your thermostat.

2. Resetting the Thermostat Doesn’t Change the Temperature

Let’s see another possibility when your thermostat, stop communicating with HVAC system. It may be really hard to pinpoint this problem, knowing how to reset your thermostat can instantly solve many HVAC issues. 
So, here your quick thermostat reset tips. A very common way is to reset almost any thermostat is to press and hold down the reset button for 5+ seconds. Alternatively, try resetting battery powered thermostats by flipping the battery directions around for 5s, then flip them back. If all these fail, switch off the power at the circuit breaker for a couple minutes. 
Try out these reset tips that work with most of the models, who knows one trick may work for you!

3. Your Home Temperature Doesn’t Match the Thermostat Setting

If your thermostat is not able to read the temperature in your home accurately, it will not be able to regulate it properly either. There could be possibilities that your thermostat is dirty from inside and it can affect its reading of the temperature. Simply remove the cover on your thermostat and use a soft brush to remove and dust, dirt or other kind of debris that may have accumulated therein. There would be a question that how to figure it out the room temperature is not as thermostat reading. 
There’s a simple way to check this out. Buying a room thermometer from a hardware store could be the answer for this. Place this thermometer near your thermostat, wait 5 minutes, and then compare the readings.

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Do both devices show the same temperature? If yes, that’s great. If there’s a discrepancy then its a problem with your thermostat. Sometime problem could exist because of thermostat location. If that’s the case, it can’t sense your your home temperature accurately. 
For this once again, you have to get an advice from your HVAC professionals on where to mount thermostat for an accurate reading and a comfortable home.