If this is a new behavior, you should check her teeth, eyes and ears to make sure there is no infection or that her teeth need to be floated. It could also be that her vision has changed somewhat. If you cannot find a psychological reason, then you should ask your vet to check your horse. Horses don’t just get face shy for no reason. If it is a psychological reason, you will have to work through it with her in a patient manner to discover what is going on. Good luck with her.