The answer to 'central air not heating' is that the most common causes are a malfunctioning thermostat, low refrigerant levels, or a problem with the compressor or other components. Symptoms can include the air blowing but not getting warm, the system short-cycling, or the compressor not turning on at all.
For minor issues like a thermostat problem, you may be able to troubleshoot and fix it yourself. But for more complex problems with the refrigerant system or compressor, it's best to call in a professional HVAC technician. They have the specialized tools and expertise to properly diagnose and repair the issue.