Have you tried playing it in a PC? Have you tried using a different camera app? Have you looked at the beginning of the file with a hex editor to see if it looks like other mp4 files? You have to diagnose where the problem is before you can begin to look for how to fix it. It could be the camera app, it could be the file format, it could be the app you use to play mp4 files - and we can't guess. Someone who has access to the phone (you or someone else) has to try things to localize where the problem is.