I seem to be getting a lot of questions lately about videos that don't work or or don't play once they've been copied to a computer or uploaded to a website. In the end, most end up being issues surrounding various incompatibilities with the video file type.
It's no surprise either, considering the massive number of formats that a video might be recorded in (3GP, MOV, MPG, AVI, etc.) and the huge number of formats among thousands of different websites and devices that you might want the video to play on.
Luckily there are several free programs, and even free online services, that will convert one video format to another:
List of Free Video Converter Programs and Online Services
I should note too that these are freeware (100% free) video converters. None put an annoying watermark on your video or trim it in any way. If you've ever searched for video converters, you know what a treat it is to find one that actually does what it says without interrupting you midway for payment!