Diagnosis
Failure in the fan speed control circuit. The triac or associated resistor on the main PCB that regulates lower voltages has likely failed.
How to Fix It
Unplug the fan. Press and hold the Power button on the control panel for 10 seconds to discharge any residual power. Plug back in. Test all speed settings (1-8) and the QuietSet modes. Cycle power off and on again. The issue is almost always a hardware failure on the control board.
If That Didn't Work
The problem is a failed electronic component on the main PCB. The fan uses a switched power supply for speeds 1-7. A single failed component disables the variable speed control, leaving only the full circuit (speed 8) operational.
Heads Up
This is not a user-serviceable repair. The entire control board assembly must be replaced. Speed 8 draws full power and is louder than the intended QuietSet operation.