Why is my heater not working

I'm assuming you mean your system is not blowing. That is usually one of two things. either the blower motor itself or the blower motor resistor. Using a test light , check for voltage and ground at the blower motor connector. If you have voltage and ground plug the connector back to the motor , turn the switch on and tap or hit the bottom of the motor with your fist . if the motor starts working, the motor is faulty. If you have no voltage at the connector, check for voltage at the resistor, If none there and you have checked your fuses , the next probable culprit would be the ignition switch. Wiggle the key switch back and forth to see if it comes on. Remember to check blower operation in all speeds. Fan speed control switches do go out but I would check blower motor, resistor and ignition switch first