What could cause sudden blurry vision?

That's an emergency! Any abrupt onset of change of vision needs immediate attention. There are multiple causes. Some include: detached retina, acute glaucoma, a stroke that can occur in various part of the brain or cranial nerves, a condition called temporal arteritis (blood vessels become infllammed), migraine... Whatever the cause, have it checked.....Today!
Acute visual blur. A quick drop in vision can be due to acute glaucoma, diabetes out of control, vascular occlusion of the eye or brain, trauma, optic neuritis, retinal detachment and other issues as well. You should as soon as you can see your ophthalmologist to find the cause and get it treated.