Sudden painless loss of vision

Loss of all areas of vision in one eye within minutes or hours or overnight often signals loss of arterial (retinal artery occlusion) or venous blood flow (retinal vein occlusion) of the retina or optic nerve (anterior ischaemic optic neuropathy).

Onset over hours to 1-2 days may indicate inflammation of the optic nerve (optic neuritis).

Preceding flashing lights or floaters in the vision usually indicate a retinal detachment retinal detachment.

Dense floaters obstructing the vision may indicate haemorrhage into the vitreous.

Simultaneous loss of vision in both eyes may be due to migraine especially if accompanied by zig-zag lines in the vision of both eyes or headache. Rarely it may be due to both eyes being affected by the conditions above that usually only involve one eye.

See also: 

Loss of central vision
Loss of peripheral vision