Hello,
Its quite difficult but can be repaired easily. First, stop experimenting with that video file of your Nokia, it may worsen the situation. I would suggest you to use Video Repair utility, that could fix your corrupt video file on your Nokia. even if the video is severely corrupt it easily fixes the file. one important point is this tool does not edit or harm the source file. it just extracts the contents from it and saves it in a new file. I feel, this tool would be the best solution for your worries about how to fix corrupted mpeg video files? first, you can go for trial version, then use purchase the software.Just give a try, Hope it works fine.

Best of luck.