This often occurs if the dog is kept shut up in a house or car without shade, ventilation or water.
It can also happen if a dog has had a long walk or running about in hot weather (remember, they wear fur coats and don't have the same sweating release that we humans have) and as a result of the dog getting overexcited or being under stress; ie at dog shows.
Signs are:
Remove dog from hot spot into cool or shady area.
Soak the dog with cold water from a hose or immerse it in a ice bath and gently massage its legs and body until you reach the vet or the animal's temperature returns to normal.
Gently dry dog with towel.
If the dog is conscious, give it small amounts of water.
Give artificial respiration if necessary.