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).
