Rear camera is not working after I dropped my phone.

It could simply be that the camera's "lego style" connector inside of the phone came loose. Therefore not producing a signal as there would be no camera installed. Disassemble the phone and reconnect it. If not that try using a magnifying glass or something else to look into the back of the camera. Make sure there is no debris in there, if so you should also attempt to disassemble the phone and clean it out.