The answer to 'why is my thermostat not heating' is that there could be several common causes, such as a faulty thermostat, an issue with the HVAC system, or a problem with the electrical connections. Symptoms may include the thermostat display not lighting up, the system not responding to temperature changes, or the HVAC system not turning on at all.
Some potential solutions you can try yourself include checking the batteries in the thermostat, ensuring the thermostat is properly programmed, and verifying the circuit breaker hasn't tripped. However, if the issue persists or you're not comfortable troubleshooting the problem, it's best to call in a professional HVAC technician to diagnose and repair the issue.