Diagnosis
The detent mechanism inside the dial—the little bumps or clicks that hold it in place—has broken or worn out. The dial is now disconnected from the internal switch.
How to Fix It
This is a physical repair.
1. Unplug the iron and let it cool completely.
2. Look closely at the base of the dial. Sometimes the outer knob can pop off if it's just a press-fit cap. Gently try to pry it straight off with a flat tool.
3. If the knob comes off, you may see a set screw or a damaged plastic stem underneath. If the stem is broken, the dial won't engage.
If That Didn't Work
The dial's connection to the internal variable resistor (the part that actually changes the temperature) is broken. The entire control assembly would need replacement.
Heads Up
With a free-spinning dial, you have no reliable temperature control. Using it risks setting it to an unknowingly high temperature and causing severe heat damage to your hair.