Rider is Down!
I am assuming if you are reading this, you are not the rider that is down. Stay calm. It will be all right.
If the rider is unconscious or you suspect great injury, call 911 immediately.
First, make certain the scene is safe. Are you safe? Is the downed rider?
Call out to the downed rider. They may just have hurt pride, get up, and get back on.
If they do not respond:
1) If you are also riding, dismount if it is safe for you to do so.
2) Secure your horse if you can. If you cannot safely do so, find a good patch of grass to let him or her graze.
3) Check on the downed rider. If you have not called emergency services and observe the need, do so now.
4) Administer First Aid if you are trained to do so. If not, make the downed rider as comfortable as possible with as little movement as possible, especially if a spinal injury is suspect.
5) If it is safe to do so, retrieve the horse or horses so that they do not pose a risk to themselves or others.