I did a bit of reading in the forums and I think it's a bug when using the video recorder. Are you trying to watch the video as soon as you finished recording it? Apparently, if you close the video recorder first and then try viewing the clip, it should play. However, and i'm not sure about this, but if you try to view it with the camera/video app still open in the background, it can corrupt the video and thus not be played later. Make sure you completely close the camera/video app by hitting the bb button and selecting close/exit before you try to view the video you recorded.
If your video is freezing during recording, there have been posts that after a minute or so the phone freezes, you can always try hitting the return/escape key once. There have been posts that it has been possible to save the video if you do that.
