Take it easy, predatory fish like that often sulk for a while before eating. How big is it? If it's a young one, it shouldn't take long for it to start eating again. If it's an adult, it might be a week or so before it considers eating again. Just don't keep dumping food in there if it's not eating, you'll wind up polluting the water and stressing the little guy out.
If the tank's large enough, and you REALLY want it to start eating again, throw a bunch of live guppies in there and just wait until they start dissappearing. In the meantime, just let him chill.