what 'hellish thing'the ancient mariner had done?what consequences he had to face

The "hellish thing" is the killing of the albatross. The consequences are myriad, and start right away. First, the wind gives out and their ship can no longer be propelled forward. The sun ramps up, and they have no water. Then the ship of Death comes floating by, and the mariner realizes his soul is at stake, though it is the lives of his fellow men that are sacrificed. The only one alive, he floats the ocean until forgiven for an act of mercy, but he spends the rest of his days in guilt, forced to tell his story to all who will listen. Killing the albatross really did him in. See the gradesaver summary for more detail.