One morning, your dog just stays on its cushion, in its basket or its kennel without the usual celebrations ...
It makes no difference, unless it is abnormally dry, crackled or oozing. Your dog may have a 40°C fever and a cool muzzle, or vice-versa. The only thing that counts is the intra-rectal temperature.
The normal average temperature is 38.5C, ranging between 38C and 39C. You need a medical thermometer, electronic preferably rather than the old mercury ones, well-lubricated and gently slipped into the rectum for a sufficiently long time. Infections are often accompanied by fever (temperature above 39C, accelerated breathing and heart-rate, and prostration). Illness may be free of fever and just as dangerous as an infection.
Your dog is uninterested by its first meal; well, okay... But is it playful, lively and alert? Yes? Then just pick up its platter of dog-biscuits, tinned dog-food, home-made meal or whatever. Do not give it something different. Serve it up again at the next meal-time. If it eats it, that´s fine. If the hunger-strike persists, phone the Vet for guidance.
What has it vomited?
With a diet adapted to its stage of life, properly wormed and vaccinated, and if the cause is not too serious (there could be so many!), it will recover with a 24 hour cure of drinking tap-water and rice-water. Resume feeding gradually. If diarrhoea is accompanied by bleeding and a generally altered condition, consult as soon as possible.
Kennel cough is highly contagious, and your dog may not have been vaccinated against this. If your dog is coughing for some other reason, your Vet will be able to find out what it is and to treat it too, shortening the duration of the cough and avoiding complications.
Siestas aside, particularly in hot weather - even if sleeping sickness does not exist for dogs, something else may be incubating. Your Vet will be able to find out what. So get along there.
With modern dog-biscuits (20% liquid), your dog needs to drink more than with tinned foods at 80% water-content. In hot weather, with exercise, water requirements go up. Never deprive your dog of water, but monitor how much it is needing each day and let your Vet know if it is excessive.