mp4 video will no longer play
Try using VLC, it plays most videos other players cannot and has a good interface. However, if this still doesn't work, try using Video Converter, Compressor and select the file then change it to an MP4 (I know it's already MP4, but this may fix any issues).

VLC: https://play.google.com/store/apps/details?id=org.videolan.vlc&amp;hl=en 

Video Converter, Compressor: https://play.google.com/store/apps/details?id=com.xvideostudio.videocompress