This may seem obvious, but you don't say what obvious things you've done....are you sure the lens does not have a film on it. Try a soft cloth (like you'd use to clean eyeglasses) to clean the lens and see if that helps. Sorry if you've already done this, and not meaning to insult your intelligence, but sometimes I've forgotten myself the most obvious fix to a problem.
If that isn't the issue, then it is likely a hardware problem and Apple will need to take a look. Does the selfie camera work ok?