It is a worrying situation when an owner finds a horse lying down and not able to stand up.  A cast horse will get muscle damage if left down for long so it is important that he is righted as quickly as possible. If you are faced with this situation, firstly summon help. 

Try to think clearly and establish why the horse may be down. Was he fit and healthy over the last few days or is he old and arthritic?  Is there any obvious injury that is preventing him from standing up?  Is he showing any signs of colic such as rolling? Can you see any physical obstruction preventing him standing or are his legs stuck close up against the wall? 

If it is the latter try to move the horse away from the wall by pulling on his mane or using long ropes looped over his feet to pull him over onto his other side. Contact your veterinary surgeon for advice if the situation is not easily resolved or if there are obvious complications.