The answer to 'humidifier not heating' is that the most common causes are a faulty heating element, low water level, or a problem with the humidistat or thermostat. Symptoms may include the humidifier not producing steam or warm air, and the unit not turning on. Solutions can include checking the water level, replacing the heating element, or troubleshooting the controls.
If you're comfortable working with electrical components and can access the interior of the humidifier, you may be able to do some basic troubleshooting and repairs yourself. However, for more complex issues or if you're not confident working with the unit, it's best to call a professional HVAC technician who can properly diagnose and fix the problem.