There are two basic kinds of hearing loss. If the hearing loss occurs in the middle ear, we call it a conductive loss. If it occurs in the inner ear, auditory nerve or brain, we call it a sensorineural hearing loss. (The old term was “nerve deafness.”)
Sudden conductive hearing losses are relatively easy to diagnose and repair. A most common example would be a typical head cold. It may result in fluid build up/infection in your middle ears. This condition normally affects both ears at the same time, and fortunately, when this fluid drains out your Eustachian tubes, your hearing returns to normal. A more serious cause could be a blow to your head, or a sudden loud sound like an explosion that could dislocate/break the tiny bones in your middle ear. Your ear specialist can often repair this damage through surgery and your hearing returns to normal or near normal again.
Sudden sensorineural hearing losses (SSHL) are medical emergencies. You need to see your ear specialist (preferably an otologist or neurotologist) immediately (not your family doctor—he is not qualified and it wastes precious time you don’t have). By definition, you have SSHL if you have a hearing loss that occurs within three days (often within minutes or an hour or two) and your hearing loss is greater than 30 dB over three adjacent octaves (test frequencies) as shown on your audiogram.