I am trying to create a character who is blind but she didn't always start out that way. What are some things that could lead to vision loss without any visible reason. Assuming this is a fantasy world with primitive technology, magic could also be sourced.
Damage to the back of the brain, where vision is processed.
There is a number of ways this may happen, including eating vegetables that have been contaminated with tapeworm eggs. Instead of hatching in your gut, they hatch in your brain.
Given the technology level you mention, your characters will not have their eyes or brains scanned for a neural cause. They probably don't even know any brain anatomy, and much food hygiene basics yet.
It used to be called hysterical blindness, but has been redubbed Conversion Disorder. It refers to a psychological state where for reasons that differ from case to case, symptoms such as blindness are manifest without detectable biological cause.
As an author, it is always fun to play with psychology when magic is available because either can explain away the other and careful writing can keep the reader guessing which is which, right up to the final reveal.
We had a student who went blind after being beaten on the back of the head in the area of the visual cortex of the brain. The doctors cannot find any visual reason for the blindness. At times it is like he sees things with his eyes but his brain does not communicate the information to the rest of the brain.
Blindness resulting from orbital complications of sinusitis. Note however that from personal experience, someone with serious sinus infection would have reduced ability to do things like concentrate for a significant amount of time prior to the problem becoming so serious as to cause loss of vision. Of course there might be exceptions, I am only talking from personal experience. Oh, don't worry; it has not progressed that far for me, but I might need a hearing aid soon.
Damage to the part of the brain that processes visual input, as others have stated.
Damage to the nerves between the eye and the part of the brain that processes visual input.
An acquired inability to produce photoreceptor cells (cones and rods).
If healing magic exists she could have had her eye damaged, waited (not necessarily by choice) until it scarred and then had healing magic cast on her which restored the eye but left the retina scarred.
You may also want to consider the possibility that she may not, in fact, be blind, but rather incapable of seeing for other reasons, for example because a spell has been placed on her which reflects all light before it hits the retina, one that places a constant light in front of the retina, drowning out all other light, or a deformation of the lens in the eye (this would cause extreme deformations in anything she sees, rendering her unable to see anything, but it would still allow her to determine if there is a lot of light or none at all).