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.
Some information about why MP4s randomly corrupt.http://aeroquartet.com/movierepair/gopro%20hero
Here is a piece of software which can re-index corrupted video files, but I'm not wild about their licensing options. But its the easiest option.http://grauonline.de/cms2/?page_id=5
Here is a free option if you are experienced in using the command line.http://goprohacks.blogspot.com/p/how-to ... ed-or.htmlhttps://www.youtube.com/watch?v=pJ4ToXDQWNs
Charles Hayden Planetariumhttp://www.thefulldomeblog.com