How to fix my iPhone 4s Front camera?

I recently got my screen replaced because it cracked, i don't know if that is it but… And what it does is when I open my camera app up, it automatically goes to the back camera and if I try to switch it, it just freezes… Also on snapchat/instagram there's no camera flip button, please help!

Try reinstalling the app. If that does not work, return the camera to the service center and complain.