Camera not working - launching into a black screen

My camera is not working, whenever I open the camera or any app that uses the camera, the app just freezes until I clear it from the background running apps. I've been facing this issue since long time. Earlier, it was a hit and go, sometimes the camera works and sometimes it does not. But since last few weeks it got totally shut down, the front camera works but the back camera only shows a black screen. I noticed many users complaining about this in the community form but did not find any solution there.
I took it to an apple authorized service center and they mentioned that it is most likely an OS issue, as there is no visible hardware damage. They mentioned that they can try re-formatting the phone, while installing the OS from their local system (not through online as it could invite more bugs).
They mentioned that the chances of this working is around 75%, however, I'll have to pay irrespective of the outcome. I want to understand if anyone faced a similar issue and got out with a solution.
Thanks in Advance!