I had that happen once with a yearling colt I was starting in a class in college. I couldn't figure out why the colt was suddenly terrified until another student mentioned the professor had use him in a "round pen demonstration" and chased him around to the point that the colt tried to jump out of the pen. I was mad, nothing I could do about it of course but make friends with the horse again.

There could be all kinds of things that could influence a horse to suddenly behave differently. She might have been chased around by a well meaning person. Or she could be in transition/season or cystic, or have ulcers. Could be silly from a weather change, or not liking the smell of your shampoo. Just take a new start and don't get frustrated (or at least don't let her know you are frustrated).

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance