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
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