There are far too many possibilities to say one for sure. One of our dogs passed away a couple weeks ago thanks to the internet man leaving under our house open. He got into rat poison and had the same symptoms you mentioned. All my other dogs were fine. If you have chicken or beef broth offer that instead of water or mixed with water. If it is low sodium even better. We fostered a pup who the doctors said couldn't be saved who had parvo once and the only thing she would eat was cat treats, she lived through it and we found a home for her. Lethargy is the main symptom of dehydration in most pets. Also check for pale gums . Think outside the box on food items to offer. You can try a small amount of raw meat (fresh of course), cat food, cooked veggies. Most of this I wouldn't do normally but in a case of eat/drink or die of starvation I say do whatever you can to make them eat. If it was me I would call the vet and see if they could work with me on whatever the reason is he can't get in today. If you explain the situation many vets will do what they can to help. If it is a matter of cost ask them if you can surrender the dog to them. It is better to have to give away a dog than it die because you can't afford to treat it. Best of Luck.