What made you think she'd sprained it? Did you see her put a leg down a rabbit hole and twist it, for instance? If not I suggest closing your eyes and feeling down that leg and the one the other side slowly and simultaneously, trying to detect differences in heat and thickness which would indicate swelling. If nothing then stand the foot in a bowl of water and clean it off thoroughly then look for footrot, scald, CODD, a thorn, etc. If still nothing then leave her indoors for a few days to let the horn harden and see if she can bear weight on it after that.