The 404 Not Found error is an HTTP status code that means that the page you were trying to reach could not be found on the web site's server.
Retry the web page by clicking the refresh/reload button or trying the URL from the address bar again. The 404 Not Found error has been known to appear on occasion even if there is no real issue so a simple refresh will often load the page you were looking for.
Check for errors in the URL. Often times the 404 Not Found error appears because the URL was typed wrong or the link that was clicked on points to the wrong URL.
Test the page at www.downforeveryoneorjustme.com. Just enter the webpage in this online tool and it will tell you just that - if the site is down for everyone, or if the problem is on your end somewhere.
Finally, if all else fails, you may want to attempt to contact the webmaster for information on where you might locate the page you're looking for.