A horse that does not want to stand up can have serious medical issues.
There is no greater pleasure than having an unscheduled half hour. Time to put your feet up, perhaps lay down in the yard and just look up at the clouds. Everybody likes that down time. Cats can be down for up to 18 hours a day. Get a dog comfortable on the couch and they will take as many hours there as they can. For horses, down time can mean something very different.
Horses are uniquely designed to be very comfortable standing. They expend almost no energy to stand. They sleep standing and find standing restful. On occasion horses decide that laying around is a good idea. If there has been a cold night and they next day is sunny, a horse might decide to lay down in the warm sun. In general, when a horse lays down, they stay up on their chest. Very seldom do they decide to lay flat out on their side. Healthy horses generally don't stay down for longer than an hour or so but there are exceptions to every rule.
When a horse lays down for longer than an hour it is time to pay attention. There are many medical conditions that can strike a horse suddenly that will make them more likely to lay down and stay down. The medical reasons for a recumbent horse range from mild to extreme life threatening conditions.
When you feel a horse has been down for an extended period of time, approach the horse. If the horse springs to it's feet and ambles off to do horse things that all is probably well. If the horse continues to stay down, gently encourage the horse to stand by clucking or gently clapping your hands. A horse that remains reluctant to stand has a medical condition and most of the conditions that cause a horse to lay down need prompt veterinary care.
The list of potential problems is very long indeed. There is the possibility of leg problems ranging from a bruised foot to a broken limb. The horse could have begun to colic and again, that problem ranges from mild to life threatening. There may be a cardiac problem. There could be head or neck trauma. It could be that the horse is sick. Many diseases, such as West Nile Virus or Equine Herpesvirus can cause severe neurological problems and render the horse unable to stand.
If you find yourself looking at your horse and realizing that he has been down for awhile it is time to go investigate. A horse that refuses to or is unable to rise means it is time to give us a call.