My Dog Won't Eat But Has No Symptoms Of Sickness Or Diarrhoea, Why?

Inappetance can be a sign of illness that is not necessarily related to a gastrointestinal upset or disorder.  I would try to tempt your dog with it's favorite foods, try offering lectade or another oral rehydration solution and make sure plenty of clean fresh water is available.  If he continues to refuse food for 48 hours, or if he starts showing any other signs of illness you should go to the vet.  Depending on the physical exam, they may have to do some tests to work out what the underlying cause of your dog's inappetance is.