If they're anything like the horns of real animals then yes, provided the bony core (the actual bone within a horn is much shorter than the length of the horn itself) is not badly damaged, the keratin part of the horn (what makes up much of its length) will grow back.And of course given we've seen techniques that can regrow a chipped tooth in seconds (albeit that was a zebra technique), chances are ponies have ways of regenerating their horns more quickly.