What do you do if your horse starts to limp?

There are too many reasons as to what would make a horse limp to possibly answer here.

A limp could be caused by a poor hoof trimming - ie: too short, a foot ailment - ie: trush or laminitis, poor confirmation, an internal leg problem anywhere up the leg ie: sprains, pulls, fractures, from an object becoming embedded in the sole, or even from something as simple as a stone bruise.