Whatsthis

After using my camera to take movies and uploading them onto my computer and play them, there is no picture coming out! There's only sound.

I used another camera, and it's the same thing. The movie's extention is .MOV but it says it's a "Quicktime movie". I tried to play it on my TV too, and they're invalid movies so, what's happening?

Thanks.

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~WhoT