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If I had to take a wild guess, it sounds to me like a codec problem. That is, 
either a folder that contains the codec you need to play the video was either 
overwritten or moved, which is a common problem when you do a software 
upgrade or re-install. If you've saved an old version of your system files, the 
correct codec should still be there.

I also use a Mac and use VLC as my main video player alongside Quicktime, 
and find it quite useful. I would recommend opening the video in VLC, 
highlighting the video in the navigator window, and choosing "Get Info" from 
the menu. This should tell you what kind of video you have (it might be an 
abbreviation that you'll have to google), then do a search of your system to 
see if you have the right codec. If not, you can often track down a Mac 
installer for the codec.

Hope this helps,
Peter