You expect your furnace to keep you warm all winter long. However, there are times when it may not do that. If it has stopped working or just doesn't work as well as it used to, there are some things that you can try before you call the furnace repair shop to have someone come out and do the work for you.
One problem that you might have is that your furnace is still putting out some heat, but it's not nearly as much as it used to be, and there isn't as much air coming out through the registers. This can be a sign that you have problems with a clogged filter. Your filter is there to keep dirt, dust, debris, and pollutants from blowing around through your ducts or from getting into your furnace. If the air filter gets clogged, then the heated air can't get through all that clog. It's going to make your furnace have to work harder to try to maintain the same temperature, but since as much air can't through the filter, the furnace just won't be able to keep up. Try replacing or cleaning the filter, and then turning on the furnace. See if that gets your heater back to the strength you want.
If nothing is happening, there are several things that could be the problem. One of the things that could be the problem is that the door switch on your furnace has popped out. Ideally, when the door is closed, it depresses the switch. When that switch is depressed, then your furnace will work. If it pops up, then it cuts the circuit and keeps the furnace from turning on. What you can do in that case is to go look at the furnace door. See if the door is completely closed. You may need to just give it a little tap to get it to close on its own. Otherwise, open the door and see where the switch is. Push the switch down so that it's depressed, and see if your furnace kicks on at that point. If it does, then try closing the door slowly and carefully, to see if the door depresses the switch the way that it should.
If you have checked things over and your furnace still isn't working the way that it should, then you should call a heating repair service to have it checked out.