My battery was replaced few weeks ago on iPhone 5s and recently my rear-facing camera & flashlight stopped working (phone reset would not help).
After pressing with thumb on camera and shaking the phone followed by a phone restart the problem got fixed.
I suspect my case it's a hardware problem. I will wait to see if this problem comes back from vibration/dropping the phone. Then will go back to the shop that changed the battery to look into it if it's their fault