Otosclerosis is a conductive hearing loss caused by plague on the ossicles.
Another type of conductive hearing loss that occurs more in girls than boys around the early twenties is otosclerosis.
This is a condition in which a sort of plaque covers the ossicles to the point where they become immobilized. This can create a severe conductive hearing loss. If it is not rectified, a person would probably have to learn speech reading and/or sign language to communicate.
Medical intervention can, fortunately, usually restore hearing to its former level. A doctor will simply peel back the eardrum (excuse me, Tympanic membrane) and insert a probe to jostle the ossicles. This will free them and restore hearing.
If otosclerosis is not treated, it can cause complications which will make treatment more complex.
NOTES: Here is some more information on Otosclerosis.