There are a number of reasons an animal may collapse. The first thing you should do after making sure the pet is out of harm’s way is to check for breathing and a pulse. If you don’t see signs of these, have somebody call an emergency veterinarian, and start CPR.
The collapse may be due to heat stroke, heart failure, shock, poisoning, or some other cause. If you have an idea what the cause is, see the appropriate section for first-aid techniques. If you believe shock is involved, put a few pellets of Aconitum 30c on the tongue with some drops of water and repeat every ten minutes while getting your pet to an emergency veterinarian.