Oh hun, it must be so distressing for you
I have found that this has happened with the boys from time to time and mostly I never figure out what caused it. I have a theory that it might have been a bad dream set it off, or something in their room scare them in the night, so the next night they don't want to go down. It usually only lasts a couple of nights in a row.
If it doesn't settle down, maybe you could try experimenting with changing the room around, leaving a light on, giving her something of yours to cuddle etc - try different things until you find one that works. GL.