The answer to 'why is my humidifier not turning on' is that there could be several common causes, such as a power issue, a problem with the humidifier's controls, or a mechanical failure. First, check that the humidifier is plugged in and receiving power. If the power is on, inspect the controls to ensure they are set properly and not malfunctioning. If the power and controls seem fine, the issue may be with the humidifier's internal components, like the fan or water supply, which would require professional repair.
If you've checked the basics and the humidifier still won't turn on, it's best to call in an HVAC technician to diagnose and repair the issue. Attempting to fix the humidifier yourself without the proper expertise can be dangerous and may cause further damage.