What is the leading reason for loss of vision?

Depends. It depends on various factors, such as the age of the patient, and other symptoms. If it's a painful loss of vision, it could mean one thing, or it could be common in old age. It can also commonly occur in a migraine. You should probably get evaluated by your doctor.
Cataract, others. Cataract, macular degeneration, gl;aucoma and diabetes are the main non refractive causes of vision loss. Good luck!