Basically, how hearing works is, your ear funnels "waves" of compressed air (that's basically what sound is) into your ear canal, then your eardrum converts those air compressions into physical vibrations, then those physical vibrations get converted in your cochlea into sensory input in the brain.

Everyone is born with hearing capacity in their brain, unless they were born with defect. Not using it doesn't mean you lose the ability to ever use it. You just have to bypass whatever isn't working (which is usually done using a cochlear implant, as far as I'm aware), and the person can hear again. They won't immediately understand what they're hearing, though; if a Hearing person goes deaf and gets an implant, what they hear post-implant won't make sense to them at first - and the same goes for someone that's never been Hearing at all, they'll have to learn how to hear, as well. They usually recommend cochlear implants before a certain age for that reason, because learning how to hear is very complex, like learning a foreign language; it gets harder as you get older.