In Oct '08, my son (then 17) was sitting in class when his ear suddenly started ringing. He said the noise started so suddenly and was so loud that he actually looked around to see who was making the noise. Within a few hours, he had lost almost all of the hearing in his right ear. At first, his regular doctor thought it might be a severe ear infection but could see no sign of infection so she sent him to specialist. Diagnosis: idiopathic sudden sensorineural hearing loss (damage to the inner ear nerves of unknown cause). Basically, something damaged the nerves in the inner ear - probably an infection of some sort but we'll never know.
The only treatment is the steroid treatments someone else mentioned. He had about two weeks of oral steroids and 6 weeks of twice a week injections into the inner ear (hard to watch them poke that needle through your kid's eardrum but he said it wasn't that bad).
His hearing did improve some but even the doctors say it might have been the treatments or it might have just improved on its own. He started with about 90db loss in all frequencies and improved to about 15db loss in the very low and very high ranges with about 60db loss in the mid range. He's probably stuck with that for life.
He had all the symptoms described by others here. Dizziness and nausea (caused by the nerve damage to the inner ear with is also part of your balance system), constant ringing in that ear. As others have said, the thinking is that the ringing is your brain trying to make up for the loss of sounds signals from the ear. The nausea went away after a few days (doc says the body adjusts quickly to that, especially when you're young). He still has the ringing but it's not too bad now.
I'm posting this mainly for one reason. If you have hearing loss and/or tinnitus caused by long term exposure to noise, well, see your doctor and protect what you have left but there is, apparently, only so much that can be done.
BUT, if it happens suddenly - SEEK MEDICAL ATTENTION IMMEDIATELY. The currently thinking is that those steroid treatments have the best chance of helping if given very early. Make sure you are talking to a specialist. A lot of people wait to seek treatment thinking it will go away on its own. Sometimes it does, sometimes it doesn't.
Here's a link:
What is sudden sensorineural hearing loss?