Your horse, if like mine at all, is probably worried she's doing something wrong, and all she's doing is trying to please you, but freezes up because she's unsure if what she's about to do is right. The best way to fix it, is to put someone on her back, and lead her around. Don't lunge first, just lead around. If you have to, coax her with food or something she likes to get her moving, and keep her moving. Practice stopping and walking again over and over. I can guarantee that eventually she will move right off. That is when you can start lunging. Lunge her with the same person on her back. It will take lots of time, but eventually she will be used to it, and you can probably then get on her back. Make sure that the person that is riding her is a very confident rider, maybe someone who's trained before. If the rider is uneasy, I can guarantee your horse won't budge. Hope this helps! :) Good luck!