This could be caused by a number of reasons:
1) The file has been inadvertantly removed.
2) Make sure that you do not have any special characters in the filename. Characters such as $%^&* may cause the browser to react differently than what was expected.
3) Make sure the file has been uploaded through a web browser using liveSite and not just FTPed into the web server's livesite/files directory. liveSite must upload the file to recognize it and manage access to it.