It may be injury or maybe there were chemicals in the sink that poisoned her. Is there any way it was there before and you didn't notice?
If she was injured or poisoned there's not much that can be done but wait and keep her water clean. Is she acting okay otherwise?
You can try adding epsom salt - pure 100% magnesium sulfate with no perfumes or other additives. Predissolve at a rate of 1 tsp per gallon and pour slowly into the bowl to avoid shock. You only need to replace salt along with a water change, and then only as much salt as water you change. It has mild antibacterial and fungal properties, and also helps with chemical poisoning.
More info could help if there's something else going on but the fall: http://www.bettafish.com/showthread.php?t=49233