The answer to 'why is my air handler not heating' is that there could be several common causes, such as a malfunctioning thermostat, a problem with the heating element, or issues with the air handler's blower motor or fan. Symptoms may include the air handler not turning on, the air not getting warm, or the system not responding to the thermostat.
Some potential solutions include checking the thermostat settings, inspecting the air filter, and verifying that the air handler's power supply is working. However, if you're not comfortable troubleshooting the issue yourself, it's best to call a professional HVAC technician who can properly diagnose and repair the problem.