This is a client-side error, meaning basically it's your fault.
You could have possibly typed the URL wrong, the URL was moved and the link you typed in doesn't redirect to the new URL.
There's not much you can do besides refreshing the page, searching the site in Google, retyping the URL, etc.
This is for the 404 Error:
"404 Error"
"404 Not Found"
"Error 404"
"The requested URL [URL] was not found on this server."
"HTTP 404"
"Error 404 Not Found"
"404 File or Directory Not Found"
"HTTP 404 Not Found"
"404 Page Not Found"