Vision loss in one eye. What causes it?

Multiple causes. If this just happened, it is an emergency and you should go to your ophthalmologist or to an er quickly. The causes include retinal detachment, vascular occlusive disease, optic nerve disease, various changes in the brain as well as other eye conditions.
Trauma + Disease. The most common cause of monocular vision loss is trauma, followed by common diseases like macular degeneration, diabetic retinopathy, acute and chronic glaucoma, and vascular occlusions of the retina or optic nerve. Amblyopia is a common cause of asymmetric visual function as well. A complete eye examination may identify the cause, or lead to other diagnostic tests. See depth perception.