I've read 24 hours but in my own experience that seems a bit short. Does your dog have any other symptoms such as vomiting, diarrhea or no stool (could indicate blockage)? Is your dog lethargic?

Your concern certainly indicates that this is unusual for your dog and you should strongly consider going to the vet. My dog recently went through something similar. Since he ate a very small amount in between, we thought it might be alright but then went to the vet since he continued to not eat. In our case, everything ended up being OK but the vet thought it was a good idea that we brought him in.

Most vets will allow you to call and describe symptoms and they will give you some advice on whether or not it warrants a visit. It's always better to err on the side of caution if this isn't something that's happened before.
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Well there are a variety of reasons as to why your dog might have suddenly refused to eat. Some of the common causes are illness, a sudden change of the meal type you normally feed the dog or a discomfort from an injury.Here is a longer and more detailed list of possible causes of a dog refusing to eat.

Infection
Viruses
Pancreatitis
Tooth decay/injury and gum disease
Disease affecting major organ systems, like the kidneys or liver
Pain from trauma and injury
Discomfort from chronic pain, such as arthritis or hip dysplasia
Discomfort due to hot weather
Anxiety or emotional distress
Bloat or Gastric Dilitation Volvulus
Intestinal obstruction

With those being the primary causes you then need to check out if any of the above are the reasons why your dog has refused to eat. If the cause is not obvious 12 hours after the refusal to eat then it is recommended you take that trip down to the vet,in the meantime I have listed a couple of terrific resources to help you find out more about dogs that suddenly refuse to eat and general dog behavior. Good luck and I hope he /she begins to eat soon,i hope I was helpful.
Source(s):
http://dog-care.suite101.com/article.cfm/why_isnt_my_dog_eating
http://dog-care.suite101.com/article.cfm/helping_a_dog_who_wont_eat
http://pets.yahoo.com/dogs/health-and-nutrition/164/refusal-to-eat/
http://ezinearticles.com/
http://ezinearticles.com/?Food-Quality-for-Your-Dog&id=1115352

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Do not over react. Dogs are smart and know when they shouldn't eat. (much smarter than people in that regard). As long as the dog is drinking I would not even consider a trip to the vet before the third day. Keep an eye on your pet and give some extra love and attention, 99.9 percent of the time they will be their old selves again in no time, all without the expense of an unneeded vet visit.
peace,
joe
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if there used to eating a certain type of dog food they don't wanna change so if u changed what dog feed that your were giving them try going back to the one before and see if the dog will eat or if your feeding it with a whole bunch of dogs then it might feel scared to eat so u could try feeding it individually. If none of them works then maybe you should just get the vet to check it out. Good luck
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Open a can of cat food for him, and if he's interested, like most dogs, he's probably just tired of what's been on offer.
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Did you feed your dog chocolate, grapes, or raisins recently/regularly? These foods are poisonous for dogs and can cause a lack of apatite. Otherwise, a dental problem can also stop a dog from eating, as it hurts to chew/swallow. Like many other members, I recommend making an appointment with a veterinary hospital.
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