This may be a camera application cache problem.
Clear the Camera cache in the settings>Application>camera. Then Clear the whole Application Cache in Settings>Storage and memory>internal memory>(swipe down to) Clear the shared application memory (or smthg like that).
If clear the camera cache cannot solve the problem, you can try reconnect the camera flex cable, cause the flex cable maybe loosen when accidentally drop or hit.
This [https://www.youtube.com/watch?v=9ZLLChEXzWk|Sony Xperia Z3 Teardown] can help you disassemble it.