The boom of the port aft crane on a jack-up rig was pulled backwards over the crane cab. The boom stayed attached at the boom base by one leg with the rest draped down outboard of the port aft rig deck. There were no injuries, no pollution and no damage to other property associated with the incident. The crane boom and live mast were severely damaged. The crane cab also had significant damage. Operations at the time of the incident were laying down the work string from the drill floor and back loading it to a workboat. The crane operator was working between two cranes located close to each other on the port side of the rig. At the time of the incident, the Crane Operator left the port aft crane cab to assist roustabouts with rigging of the pipe that was to be back loaded onto the workboat. The crane was left unattended with the engine running and the boom positioned over the pipe deck at about a 65 degree angle.