The answer to 'humidifier not turning on' is that there are several common causes, including a faulty humidifier control, a problem with the power supply, or a clogged or dirty humidifier. If your humidifier is not turning on, you'll likely notice that the unit is not producing any moisture, and you may see that the indicator lights are not illuminated.
Some common solutions include checking the power supply and verifying that the humidifier is plugged in and receiving power, cleaning or replacing the humidifier filter, and checking the humidifier controls to ensure they are set properly. If you've tried these basic troubleshooting steps and the humidifier still won't turn on, it's best to call in a professional HVAC technician to diagnose and repair the issue.