You've fixed the water leak or other cause of water damage, but that water stain is still there. You might not have to repaint or fix the ceiling, though. Try spraying bleach on the spot.
Photo by Ed!
Family Handyman recommends spraying the spot with a bleach and water solution (10 per cent bleach). In a day or two the stain might disappear. If it's an old stain, however, they recommend using a mould and mildew remover instead. Protect the walls and floor -- and your eyes -- of course.
10-Minute House Repair and Home Maintenance Tips [Family Handyman]