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.