Had the blank (white) screen problem first then the camera(s) issue. Blank screen fix is hold Power, Up Volume and square button at same time til Apple appears then restart.
I hadn't removed the cameras during screen repair, so I went back to check all connections as it sounded like a hardware problem for sure. Found the front side camera plug loose on back of mother board; made sure the "little" spring was not bent and also found the new screen did not have the metal plate glued to it like the original. The spring makes contact with this metal tab which also grounds to the copper foil which wraps the digitizer plug.
Used strips of carpet tape (cheap and very sticky) to put all back together; both cameras working. Hope this helps.