iPhone 4s front camera not working
The camera's probably just stopped working and needs to be replaced. Check out ifixit.com for a step-by-step guide on how to replace it and then decide if you feel comfortable with the doing the repair. If not, there's probably someone in your area that can do it for you.

If it were me, I'd save any money I'd put towards a repair, back the phone up to iTunes, save the photos to my computer, and just upgrade to a newer iPhone.