Please confirm your device and your video player app are compatible with the file type/encoding that you are attempting to play.   Both Apple & Andriod devices will only playback file formats that are supported by their respective systems.  There are other, 3rd party video players that can be installed on your Apple or Android device that will allow for broader video playback compatibility.   You may also need to confirm that the video file has not been accidentally corrupted by playing it back on another device.