What are the common causes of sudden loss of vision in the lower half of one eye?

Optic neuropathy . This is a classic symptom of ischemic optic neuropathy although it usually occurs in the upper visual fields . Other possibilities include retinal detachments , arterial occlusions of the retina, infections, and autoimmune disease of the retinal vascular system. The list is extensive and this just names a few .
Vascular, detachment. Loss of vision horizontally and only in one eye suddenly usually means a stroke of the eye where one branch of the retinal vessels is blocked. Due to the vascular anatomy this makes a horizontal line. Occasionally a superior or inferior retinal detachment can cause similar vision loss but this is usually slower and progressive.