If you have a dog not eating you are probably concerned and actually you have good reason to be. Dogs losing their appetite can be caused by simple behavioral or environmental reasons that can be fairly easily addressed at home. However a loss of appetite could also be a symptom of a much more serious problem that is going to require veterinarian care.
Let's talk about the serious side first. A dog not eating, particularly if he or she has other symptoms like vomiting or constipation or passing blood in urine may be suffering from one of a number of serious conditions like bloating, liver disease, kidney failure, dental problems or even cancer. All of these require immediate veterinary care as they can all be life threatening.
Dogs are pretty durable animals and can easily go a couple of days without eating with no ill affects. However if your dog was a good eater and just suddenly stops, you need to see a vet. This is particularly important for puppies and older dogs. Once the underlying cause is determined, your canine pal can be treated and get back to health.
There are other reasons that a dog may stop eating and these have nothing to do with disease.
If there has been a change in the environment, like a move to a new home or apartment, or even a new eating place within the current home the dog may not have made the adjustment yet.
If your dog was normally fed by himself and is now being fed with other animals, or thee reverse, there has been a change in what he or she believes is the normal routine and this might put him or her off her feed.
Vaccinations are important but occasionally there are unwanted side effects. If you dog stopped eating shortly after receiving a vaccination, check with your vet and see if that is a known side effect.
With older dogs it might be that it simply hurts to eat. There may be problems with the teeth that make it hard to chew. If the dog has developed arthritis in its neck, it may be painful to lean its head down to get the food. In this instance try using a phone book to raise his bowl and see if that makes a difference.
Then there is the picky eater. Just like humans, they can be a pain in the butt. You can try mixing up the type of food but the best approach is to simply designate feeding times. The bowl goes down for an hour twice a day and that's it. When the picky eater gets really hungry they'll eat.
So if your dog is not eating try to figure out if it's physically driven or just behavior. If you think there may be an illness behind it seek help from the vet right away. If not, it's just a matter of getting the dog adjusted.