What could have caused a sudden blind spot in my left eye that impaired about 40% of my vision?

Need eye exam ASAP: There are many possible causes. If the blind spot was temporary and the visual loss completely resolved, this could be a visual migraine (which usually affects both eyes) or even a transiet ischemic attack (TIA) - which is much more serious and should be evaluated immediately. If the loss remains, retinal problems such as a vein occlusion, retinal detachment, or an optic nerve problem are possible.