Loss of appetite can be a symptom of illness. If you notice any other symptoms or changes in your dog's usual behaviour you should contact your vet. Your dog may not be eating if he has a problem in his mouth that makes it painful to chew. If this is the case you should take your dog to the vet. If your dog has just had vaccines or an operation it is likely he will lose his appetite for a while, but it should return fairly soon and usually isn’t anything to worry about.
Another possible reason for your dog not eating is if you have changed his food. Sometimes dogs won’t eat if their surroundings change, and it may take time before they will eat as they adjust to their new environment. Some dogs can be fussy eaters, and you may have to try a few different foods before your picky pup will eat. You also need to consider if you are giving him too many treats, too many treats throughout the day may affect your pups appetite at dinner time.