Front camera on an iPhone 4S no longer works
Thanks to @IanC to find the duplicate, so before we close this here is the answer.

https://apple.stackexchange.com/a/61350/46541


  This is a hardware problem. This comes up when the ribbon connector
  from the camera to the main circuit board comes loose or gets
  disconnected altogether. If you have the right tools and enough DIY
  skills to open up your iPhone, it's a very simple fix that should be
  fairly obvious to see. Otherwise, the Apple Store should be able to
  repair it for you (I would think free of charge, but who knows).
  
  If you do want to take apart your iPhone and attempt the repair
  yourself, PowerBook Medic has some really nice instructions for taking
  apart all versions of iPhones from 3GS and above. They also sell
  replacement parts and will repair it for you as well if you want.
  (Disclaimer: I have no affiliation nor prior experience with this
  company, so use at your own risk.)