MP4 video changed to jpj file

I took a video of my kid riding his bike for the first time.

The video worked great and I showed it to a few people.

The last time I showed the video to someone the file turned into a picture (JPG).

Now it is just a black picture.

I have used ES exployer to change the file back to a .mp4 but it says the cannot open the file. I also tried changing to an AVI.

I have also re-started, taken out battery for 10 seconds ect.

My phone is the S3 SGH-1747M, 4.1.2.

I have searched but did not see anything simliear to this.

Any help is appreaciated.

Thanks
Dean