What would cause sudden hearing loss?

May be simple. May be related to something as simple as ear wax impact ion. See your doctor.
Still an enigma. Sudden deafness that occurs in 24-72 hours, can be due to a viral infection, vascular occlusion, auto-immune disease and head injury for example. If seen by an otologist early, intra-tympanic injection of steroids or high doses of oral steroids may help. There is also some evidence that sudden deafness may be a genetic disease.