Quicktime X is a great little piece of software. However, Quicktime X is very finicky on which codec/file format it can decode/play.
Luckily, there are DOZENS of alternative video/media players and extensions available for the Mac. Some of them are even FREE. Some of the better FREE ones include...
PERIAN (quicktime extension)
http://www.macupdate.com/app/mac/22923/perian
QUICKTIME 7.6.6
http://support.apple.com/kb/DL923
VLC Media Player
http://www.macupdate.com/app/mac/5758/vlc-media-player
NICEPLAYER
http://www.macupdate.com/app/mac/15136/niceplayer
... And if none of those work, there is a possibility that, that video file may be corrupted.