Hi!
Try the usual first. Try a reset. There should be a process for the iPod touch that will interrupt everything and either shut it down or restart it. Once restarted, try viewing the video.
If that doesn't correct the problem, sync it with your computer. First, eliminate the "bad" video, and sync it with out the video. Then add that video to the list of what you want on it, and sync it again. After you disconnect your iPod, try viewing the video.
You should also check out the Apple website and look for iPod touch support. See what they have to say about common problems.
Good luck and let us know how it's going.