What To Do
Call your vet immediately. You can look for an injury and assess your horse's general health using the Whole Horse Exam (WHE), but do not force the horse to move. Pay particular attention to the rectal temperature and heart rate. Look for swelling and heat in the limb but recognize that it is very hard to compare a non weight bearing limb to the supporting limb. Feel for digital pulse and gently lift the limb to see if this causes a pain withdrawal response. Allow the horse to stand still quietly until your vet arrives. Talk to your vet about whether you should medicate your horse with an anti-inflammatory/pain reliever to relieve pain until they arrive. If you do this, tell your vet exactly what was given (drug, time, amount) because it may change their interpretation of their examination.
What Not To Do
Do not force a severely lame horse to walk more than a few steps unless absolutely needed or at under vet direction.