What is the cause for sudden deafness?

From the sketchy details that you have given, it appears that you wife might have had a block of the blood vessels that supply the inner ear. However, the mechanism of hearing is such a delicate one that any such speculation may not be correct. You have stated that the catastrophe occurred 28 months ago. I presume that relevant investigations would have certainly been done. Does your wife have any diabetes or hypertension? Was there any other associated symptom of giddiness or a sudden fall? Or a preceding fever? Had any brain scan been done to rule out any tumour inside the brain? Did you get an x-ray of the neck bones done? Have you done a carotid doppler study to rule out a block of the main blood vessels reaching the brain? If the above investigations were done and a neurologist's opinion obtained, you can rest assured that the condition may not progress further or that the condition does not presage any other disease. Otherwise, kindly get a personal consultation / review by an otolaryngologist (Ear Nose Throat - ENT specialist) and a neurologist.