That is very odd, But then again it can happen if it is a large file.
To be able to view it, you could try connecting the phone to a computer, via USB, and then moving the video over to a folder on the computer.
You would then be able to see what the file extension is, and see if it is viewable on the computer, or convertible to another format (through programs like "Format Factory").
In the end, however, it is completely possible that the it was somehow corrupted by another app (or something worse) that is on the phone. But hopefully you will be able to view it or convert it to view on a computer.