Horse is Down!
Call the Vet immediately if your horse is down and refuses to get up.
Could he or she be sleeping? Don't panic. Try to gently and safely wake, then louder if necessary.
Is your horse conscious? If not, hang in there until the vet arrives. If you can safely do so, try to take vitals.
Is your horse rolling or biting at his or her sides? Possible Colic.
Are they just laying there, but awake? In distress?
If they do get up, proceed to check vitals, check for gut noise, and check for heat in feet and/or a bounding digital pulse.
If the vet is on the way, stay with the horse. Do not allow it to eat unless advised to do so. If colic is suspect, keep your horse walking to stimulate gut. Note if the horse poops.
If you have decided to not seek the assistance of a vet, you may consider monitoring your horse for at least another hour or more after they are up again to be certain it is non-emergent.