DIAGNOSIS This is almost always a power supply or unit assembly issue. The unit isn't getting power or a crucial internal connection isn't made.
FIX First, check the power cord is fully plugged into the BACK of the unit, not just the wall outlet. This cord is detachable and can look seated but not be clicked in. Push firmly until you hear/feel a click.
IF THAT DIDN'T WORK Ensure the water reservoir is correctly installed. It must be pushed all the way in until it clicks and the float moves freely. An unseated reservoir can prevent the unit from powering up.
HEADS UP Don't run the unit without the reservoir properly clicked in—it can cause water to leak into the electronics.