Ditto what ginger said. Also, if your dog is not drinking, that's serious. You can't risk dehydration, so keep a sharp eye on how much water he's taking. You might try some low-sodium chicken broth in the meantime, but as ginger mentions, if he's not better in a day it's definitely time to see the vet.
Has this happened suddenly? Is his appetite usually good? Have you changed his food?
It could be anything, from a simple dislike of his food, to a stomach upset....to something really more serious. Have there been any other symptoms -such as vomiting, diarrhea, coughing or retching, unusual tiredness?
Any dog who normally has a good appetite and isn't too faddy with food, will eat. A sudden change in appetite usually means your dog isn't feeling well. He could have some pain somewhere. Pain can cause a dog to lose his appetite.
If he doesn't start eating within 1 day or if there are any other symptoms at all, I would take him for a good check up with the vet.