I am not deaf and nobody I know is, yet I am curious how an author goes about writing in a newly-deaf character's POV when they aren't deaf themselves.
Research is surely the way to go.
An even deeper alternative would be to lose your hearing, not forever of course. Just wear protective earplugs or some gear of that sort, and try interacting with your family/friends for more than a week. You'll get first hand experience to how it feels to be suddenly isolated from the world of sounds. You may vocally get loud during this course of time and experience a lot of other feelings from within.
But the catch is you can't take them off midway. Suppose you want to talk to your girlfriend or boyfriend — you have to play along, you can't take small breaks from it.
This would definitely give you a deeper level of understanding and emotional transition and experience the helplessness that a person feels.
All the best, do update on the approach you take.
There's no substitute for research. Either find a deaf group in your area or contact a national group, or possibly Gaulladet University, and start talking to people.