Unfortunately it is kind of hard to narrow down what the issue could be. I see a lot of times that when any of the cameras or flash quit working all three will stop working. Since the area that was smashed was where the back camera is, I would say it is safe to assume it it either your back camera or flash causing the problem. Though it could, of course, be a board issue, I would say the chances of that are slim. Also as long as the back on the phone is not damaged, most likely the flash will be fine as well. So, most likely you just have a broken back camera, and it needs to be replaced to fix the function front camera and the flash as well. Happy Fixing!