This one is a common error. Try deleting everything after the last slash in the URL to see if you get a page with a link to the page you want. Also, check your spelling. Sometimes, however, the pages are just missing. Wait a minute and try the page again.
Chances are strong that the page is there.
Try looking for it with a search engine. Also, if the URL you entered is long and complicated, guess how it may be simplified. Check to see if you lost your Internet connection. This happens most often if you've gone away from the computer for a period of time. If you aren't connected, log back on and retry the site.