So this is new behavior, she's never had a problem with it before?
First, has she been "behind" on turnout... they can do this sometimes if they are feeling really pent-up and stroppy. Also is she in heat, a few mares can be really weird then.
Do you have electric fence. That would be my next thought. What you describe is CLASSIC stray voltage or short or fault-in-buried-wire behavior. If you have a hotwire buried under a gateway or driveway, that is the likeliest suspect, but a wide variety of other faults can give similar results. Animals may refuse to go near the zappy area even once the fence charger is turned off, as they've learned "that gateway has evil feet-zapping fairies!" (or "that water tub" or "that gate" or whatever).
If you have no electric fencing on the property, the major remaining possibility that I can think of is that something has scared her around the barn. An animal that is (or was) in the barn; a piece of roof tin flapping in the wind just the wrong way; she knocked down an iron pipe leaning against the fence and it scared the bejeebers out of her; that sort of thing. You can't always figure out what it was, you just have to deal with it as a training issue. With some horses a bucket of grain (held by you or by a second person in front of you) can solve most "won't, can't, shan't!" type problems; with other horses it can just make them more obnoxious. I would suggest working not just on getting her back to the barn but on the whole back-and-forth routine, preferably in many short sessions with some reward she likes, rather than as a single marathon and she feels there wasn't anything in it for her.
Good luck, let us know how it goes and if you figure it out,
Pat