I am sorry to hear that Luna seems to have a one of her eyes rolled back into her head.
Does her eye look like the dogs in the pictures in the links below?:
If she looks like one of those pictures then her eye isn't truly rolled back, rather the protective third eyelid is raised. The third eyelid is a thin membranous protective eyelid that normally sits hidden in the nose side corner of the eye.
It generally rises in response to some sort of irritation, whether it be a scratch or foreign body in the eye, an infection or an allergic type reaction.
Sometimes we see it come up in both eyes in dogs that are very sick systemically, dehydrated or very malnourished.
If the pupil were smaller then the other, unaffected eye, her eye itself looked sunken and smaller compared to her normal eye, and her lid looked droopy then she may have a condition called Horner's Syndrome. This is a nerve problem in that the nerves that control keeping her eye open and the third eyelid down aren't functioning normally. The nerves that control those functions are long, leaving the brain, traveling down the spinal cord into the chest, leaving the spine there, entering the center of her chest and then traveling all the way back up to the head, passing by the ear. Thus there are lots of places where inflammation/pressure on the nerves can occur affecting the nerve function. The most common causes in young otherwise healthy dogs are idiopathic, meaning we don't know why it happens, or related to an ear infection. The second dog pictured has Horner's syndrome. Notice how small her affected eye looks as well as the third eyelid rising.
If her eye size looks normal, not drawn back into her head compared to her other eye, and her lids aren't droopy, and because only one eye is affected I would be suspicious of some sort of trauma (possibly from another pet or plant material?) or a foreign body (grass awn, grass blades) in her affected eye.
If she is rubbing her eye and/or squinting that does usually indicate discomfort/pain. I would recommend placing an elizabethan collar (lampshade, cone of shame collar) on her so she cannot cause any further damage to her eye by rubbing it.
Perhaps being a curious pup she stuck her nose somewhere into plants and scratched her eye or got some plant material in it.
If she is feeling fine otherwise there are a couple things that you can try at home. You can purchase all of the things I mention below over the counter at the local drug store.
You can start with flushing that eye copiously with sterile saline, that should soothe her eye and hopefully remove any sort of foreign body.
Then apply artificial tears every few hours or the next few days to keep her eye cushioned and hydrated.
Apply a small amount of Polysporin ointment 3 to 4 times daily to her eye for 4 to 5 days to prevent an infection.
I do recommend trying to keep her in a low light setting, at least for the next 24 to 48 hours.
If this is a mild scratch then it should start significant healing over the next 48 hours and her third eyelid should regress.
If she isn't much better in a couple days, or her eye becomes swollen or really red or she is pawing at it, she should be looked at by her veterinarian to try and diagnose her particular problem.
Best of luck with your girl, please let me know if you have any further questions.