"Sudden hearing loss" is a recognized thing. Sudden is right; you can go to bed with normal hearing and wake up deaf. It's probably viral. It usually occurs in just one ear. There's not much that can be done to treat it, viruses being viruses; when I went stone deaf in my left ear, the docs gave me steroids to boost my body's general recuperative powers but then, as with other viral infections, you have to rely on the body to fight it off itself. In most cases, including mine, people recover most or all of their hearing. This can take weeks or even months. Sometimes there is permanent damage ranging up to permanent loss of hearing.
The docs are handicapped in studying it because it's an infection of the inner ear, which is not an area that is open to easy observation. I had never heard of it until I got it, whereupon I was swiftly informed that it's much more common than most people imagine.
I had double vision for about 2 weeks. No known cause, the eye specialist said it was kinda like a stroke only muscle-bloodflow, not brain-bloodflow. Returned to normal.