I would check and make sure the video you uploaded is physically in the /media folder.
Second, make sure the [video] tag exists in the post. It'll look like:
What is the extension of the video file, btw? Is it .mpeg4 or .mp4?
Try navigating directly to the video in your browser. The URL will look something like:
If the video starts playing or you get a Download/Save As dialog window, that is good. But if you get a 404, Page Not Found error, and the file does exist in the /media folder, this probably means the MIME type for the extension is not setup on the
server. If you're using IIS7, you can add the MIME type yourself in your web.config file (the one below is a MIME type for the .MP4 extension).
<mimeMap fileExtension=".mp4" mimeType="video/mp4" />
If IIS6, the MIME type has to be configured directly in IIS.