Try redownloading and reinstalling a new, fresh version of Quicktime from
www.quicktime.com. Often this solves the problem right off the bat. If that doesn't work, don't worry, there's a plan B.
If reinstalling Quicktime doesn't do the trick, try downloading and installing a free player called VLC from
http://www.videolan.org/vlc/. This is a slick media player that can play just about any format of anything. It can even play VHS tapes and N64 games on your computer. I might have just made up that fact, but it's possible. VLC is available in PC and MAC format, so either way it should help you out.
After downloading and installing VLC, try to play the video in there. If it works, please let me know so that I can feel proud of myself. If it doesn't work, please pretend like it did so that I can still feel proud of myself. Just kidding. If you still have any issue after trying VLC, submit a support ticket and our support guys will take good care of you.