DIAGNOSIS You've got a classic auger jam. A piece of foreign material (a rock, a weirdly shaped pellet, a wood chip) or a mass of damp, sawdust-fine pellets has locked up the auger motor.
FIX Unplug the grill. Empty the hopper completely. Use a long, flexible grabber tool or even a straightened coat hanger (carefully) to try and dislodge the blockage from the hopper side, working it down the auger tube. You can also try manually reversing the auger by accessing the motor and turning the shaft with a wrench.
IF THAT DIDN'T WORK The shear pin on the auger motor shaft might have broken (it's designed to). You'll need to take off the hopper to access the motor and replace the small metal pin. It's a cheap part.
HEADS UP This is why you should always pour pellets from the bag into the hopper, not dump the whole bag in. You'll introduce all the dust and debris from the bottom of the bag straight into the auger.