Diagnosis
The dial is likely disconnected from the internal valve it's supposed to turn. It's a known weak point on this model—the plastic linkage between the knob and the variable pressure regulator can strip or break.
How to Fix It
You'll need to open the handle housing. Remove the screws (usually under rubber grips) to access the internal assembly. You'll see a white plastic gear on the dial shaft that should mesh with a sector gear on the pressure valve. If it's stripped, the dial spins freely. The fix is to reassemble it so the gears engage, or if parts are broken, replace the handle assembly.
If That Didn't Work
If the internal mechanism seems intact, the issue could be with the pressure regulator valve itself, located in the pump. That's a more complex repair.
Heads Up
While broken, you're stuck with whatever pressure the pump defaults to (usually max). Be extra careful with the spray, as you can't turn it down for delicate surfaces.