Horse just turned up lame?

Does her foot feel soft anywhere? Hot? It sounds like it could be an abscess. Try soaking her foot in Epsom salts dissolved in water, then wrap her foot in vet wrap and either duct tape or an easy boot. Soak for 20 minutes every day, and see if that helps. If it's an abscess, it should take only a few days to a week to heal up. If she doesn't get better with the soaking, it could be a hot nail, or a stone bruise... In that case, I'd call the vet.