It's sometimes, thus, a bit of a concern if a horse is unexpectedly down - and one of the worries is that the horse may have become "cast."
A cast horse is one that has lain down and is now unable to get up. This can happen if a horse attempts to lie down in a standing stall, but is most commonly caused by lying down too close to a wall or a fence. The horse may end up in a position where its hooves are between its body and the wall and it can't push them out to get up.
An arthritic horse may also become cast because its too stiff to move its legs in the correct direction - and if this becomes a regular thing it may indicate that euthanasia is recommended (horses can and will die if they stay down for too long).
A cast horse requires assistance to free itself. Usually, this means gently wrapping ropes around the horse's legs and pulling it over so that its feet are pointing away from whatever obstacle it has got itself wedged agains. A horse stuck in a standing stall may require using the stall walls (which generally don't go all the way up to the roof) as pulleys and sliding ropes under the horse's body to literally pull it up - a process which generally takes three or four people. I've had to do it once and I never want to do it again. Ever. I think it's the most physically demanding equine task I've ever been involved in.