[Stepper motor jammed.... makes buzzing noise but shaft doesnt rotate]

The x axis stepper motor of my UM2 is jammed. It only makes buzzing noise but the shaft of the motor doesnt rotate.

the other motors are working fine.

Is there a way to trouble shoot this problem??

There cannot be a mechanical obstruction inside the motor because it is completely sealed.

i tried to loosen the pully attached to the shaft but the hex bolt is extremely tight and the key keeps slipping.