The 5.8 system should be pretty reliable. The basics are to have someone heave on the halyard to raise the sail slightly above the hook. Then rotate the mast hard one direction (you really need to get it to 90 degrees of rotation). Then, with the mast still rotated, pull down on the sail and hopefully it's unhooked. If that didn't work, try again but rotate the opposite direction. If that still doesn't do it, take a close look at your knot and do as Dave suggested and reverse it so the bulk of the knot is on the other side of the ring.