Sometimes an MP4's can corrupt in camera and the header isn't written correctly in the video. This has nothing to do with the memory card. Try watching the MP4 in VLC and if the video is completely black throughout, then you will need to fix the corrupted video file by re-indexing it.
If you accidentally remove the memory card while the camera is still recording, then this software is the best option to fix the corrupted video file. (Thanks to Eric Fassbender for suggesting this tool and Ryan Whitehead for confirming it works.)