A common problem when using jwplayer is "file not found or access denied".
Here are a few things to check for:
1. If you are using mp4, be sure to have the MIME type added in IIS. The MIME type for .mp4 is video/mp4. The video will work in a test html page without a MIME type, but you’ll need to add one when working with ASP.net
2. Be sure the file location is correct and that the file exist and is named correctly. You may want to try using an absolute web path name inside the javascript for testing purposes.
3. Check your relative file path. Some versions of the player (v5.3), a relative path must be relative to the SWF file, other versions, the relative path must be relative to the html which the SWF is embedded (v5.4) . See here http://www.longtailvideo.com/support/forums/jw-player/video-encoding/12978/mp4h274-video-not-found-or-access-denied
4. Be sure you are using forward slashes instead of backslashes in the file name
5. View the source of the javascript to confirm it's correct.