THE LATERAL (SIDEBOARD) HAD COME OFF THE PLANT CONVEYOR BELT AND COWORKER TURNED OFF THE CONVEYOR TO LOCATE THE LATERAL. EE PROCEEDED TO ACCOMMODATE THE LATERAL (PICK IT UP & POSITION IT ON THE BELT) BUT EE DID NOT NOTICE THAT THERE WAS A STONE FALLING. THE STONE STRUCK EE BETWEEN THE NAPE OF THE NECK, HEAD AND SHOULDERS.