Some possible illnesses that come to mind are a stroke, inner ear infection, tumor (most likely in the brain).
Over the years this has happened to a few of my rats, hamsters & gerbils and it was always one of those problems.

Is she able to move that side of her body? If she can't or if it is limited it is most likely a stroke or tumor (probably brain tumor).

I hope you can find a vet. Good luck with her, she's in my thoughts.