Why is my heater not working

Is the car overheating also? if not, most likely you have a stopped up heater core.

You might be able to get it to work again for a while by flushing the heater core with a garden hose.

Please be aware, do NOT do this when the car is warm/hot. You could burn yourself or damage your car if you do

Most likely you have at least 4 hoses for the cooling system on the car. (you could have more) two going to the radiator, 2 going to the firewall. and maybe more going to the carburetor or throttle body. Find the 2 hoses going to the firewall and make sure the car is cold, Trace back the hoses from the firewall, remove the hoses from the non firewall end and then while disconnected take a garden hose on somewhat low pressure and run water in the hose and look at the water coming out of the other hose (if you put the hose end in a bucket, you can see the crap that is coming from the heater core) run water in to the heater core until you get clear water out of it and then put the garden hose in the other heater hose and flush until clear. Do this 3 or 4 times, put it all back together and refill with anti-freeze. This is a MESSY job but is easier and cheaper than replacing a new heater core