If your furnace isn't working, you are likely to need a professional technician to diagnose and fix the problem. However, before you make the call, there are a few things you should look at yourself. These are basic things that don't require any expertise in heating systems, that most people can do themselves. The following are a few things to try.
Check your thermostat
The first thing you need to do is check your thermostat. Obviously, you should see if somebody hasn't tampered with the settings, but if you have a thermostat that operates off a battery, you should check if the batteries are working. Often, the display will say low battery, some thermostats will display nothing. If you're not sure, then go ahead and replace the batteries. If you have an electric thermostat, then you need to check your circuit breaker to see if it popped. If it has, you need to reset it. But if the thermostat pops again, you may need to call an electrician and not a furnace repair company.
Check all the vents in your home
If it takes longer to heat up your home than normal, sometimes one or more of the vents could be closed or obstructed in some way, such as objects on the top shelf of a bookcase. Make sure you check all the vents in every room of the house.
Check your air filter
If you haven't been changing your air filter according to the furnace manual, this could easily be the problem. Usually, the furnace will be operating, but it won't be putting out enough heat. Look at your filter. It will be obvious if you need to change it. If you do need to change it, go ahead and do so, even if the problem turns out to be something else.
Check the pilot light
If you have a gas furnace, this is a common reason it isn't working. Usually, your furnace will be operating, but it will be blowing out unheated air. You need to check where the pilot light is in the furnace and then light it. But make sure it's done properly by your manual. Although it is easy to do, don't do it the wrong way because it may be dangerous, so make sure you read your manual to determine the proper steps.
You can also check your owner's manual for additional things you can fix that don't require any technical knowledge. But once you've made all the basic checks and done any minor repairs that you could do on your own, reach out to a furnace repair technician. Most of the time, this is what you need to do, but it is worth checking for basic issues on your heating system.