WHen I'm in a pinch (havent' had that exact thing yet though) I use a free meida player that plays almost anything: VLC. http://www.videolan.org/vlc/
Before you do that, ahve you rebooted? I know that sounds obvious but I run into that where I work a lot and a restart is all it takes to get a computer to work again.
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A Parent, old geezer, and occasionally right. Good judgment comes from bad
experience, and a lot of that comes from bad judgment.