One possibility is that the site updated and the bookmarked URL is obsolete. Another possibility is that the site protects the page from direct access by requiring the browser to provide an appropriate referring URL or current cookie with the request.

If you go to the site through its main page, can you get to the page you want to reach? If it has a new URL, you just need to update your bookmark. If it's the same page, hmm, could be a security thing.