Try redownloading and reinstalling a new version of QuickTime Player from
http://www.apple.com/quicktime/download/. Often, this solves the problem right off the bat.
If reinstalling QuickTime doesn't do the trick, try using the VLC Media Player from
http://www.videolan.org/vlc/, which is a slick media player that can play just about any format of anything for both PCs and Macs.
After downloading and installing VLC, try to play the video in there. If you still have any issue after trying VLC, submit a support ticket directly at
http://www.theory11.com/support, and the support guys will take care of you.
Cheers,
JTM