Lameness. Check to see if the horse is "off" or lame in the affected limb. Any lameness you find may not be related to the hoof: If he lost the shoe while pulling free an entrapped hoof, for example, the struggle may also have caused trauma to sensitive structures within the foot or even injury higher up on the leg. A minor strain may heal with a day or two of rest, but significant or persistent lameness is best investigated by a veterinarian.
Bleeding. As a shoe pulls off, it sometimes damages the hoof around the nail holes, leading to anything from minor cracks to the loss of hoof wall sections. If you find blood around any of these defects, the wound may extend to the underlying soft tissues, and infection becomes a serious risk. Call your farrier or veterinarian if you horse's hoof is bleeding.
Instability in the hoof wall. Even if cracks in the hoof wall appear to be minor, walk the horse on a smooth, hard surface and watch to see if any portion of the horn spreads or shifts as that leg bears weight. If so, the crack is deeper and more serious than it looks. Restrict the horse's movement until a veterinarian arrives and stabilizes the foot.
Puncture wounds. If your horse stepped on the shoe as, or just after, it fell free, the nails could have punctured the sole or frog, which could lead to serious infections of the deep tissues within the foot. If you see any wound on the bottom of the hoof, no matter how small, call your veterinarian. If there's a nail protruding from the sole, leave it in place and keep the horse still until the veterinarian arrives.