Why is my Front and Back camera not working?

My front and Back camera have stopped working, only a black screen with all camera options appear when camera app is opened.

Findings:

Camera is not damaged.

Not a software issue - as iPhone has been reset and restored to stock iOS.

Camera replacement did not work.

All the other functionalities on the phone apart from camera works like charm.

Please let me know what else can i do to resolve this.