I have a front wheel drive self propelled mower that the front right wheel is frozen.what could it be?

Hi,
Most likely cause is something has gotten past the dust shield and is jammed between the pinion and wheel gears. Nearly as likely is dirty, rusty or corroded pinion/pawl combination.
With the first you can often lift the front of the machine and physically free it temporarily by turning in reverse direction.
At any and all rates, the wheel/tire must come off to repair and the problem will likely be more obvious.
Peace,
Clarke