Thanks a lot for your responses

The vet thinks Star somehow hit her head very hard, possibly in the stable the night before. She has no bruising on her face or head but I guess she could have hit her poll maybe when having a little roll in the stable.
The whole left side of her face is affected, including the left ear. I'll make sure to lightly massage her face every day, she has a little feeling in it and has been rubbing it on her foreleg the odd time.
I'm not allowed to ride her until after the bone has healed which could be around Christmas time. We have to go back to get an xray in Dec to check it has healed before starting gentle work again.
The corneal ulcer is quite visible, so I'll keep a close watch on it.
Thanks again for your advice