How to retrieve a halyard from masthead???
Jeff has probably given you the best ideas but if neither are practical, try looking over the situation through your binoculars. Make a strong wire hook and attach it to the top section of 4 - 10 foot pieces of 1-inch PVC pipe and try running it up, tying it periodically to the headstay. It may require a few shots to adjust the hook properly but using your binoculars, you may be able to hook on to your halyard and haul it down.