Turn your monitor upside down. Else stand on your head. lol

If it happens to more than one video you've tried then I'd guess it might be a CODEC issue.

* A list of CODECs sources can be found here (choose one): 1. Free-Codecs.com, 2. Nimo’s codec pack, 3. CoDecs & codec identifiers, 4. K-Litecode Codec Pack

* Get GSpot & open the video up with it & it will tell you which codec you need to play it: http://www.headbands.com/gspot/

* Get a backup player with in-built codecs & is very reliable such as VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)