A 404 error page comes up whenever someone clicks a broken link, or goes to a URL on your site that doesn't exist. To make a custom 404 error page you must first make a page that you want to be your 404 error page. It can have images, CSS, a message about the error, etc... Name this page 404.html, save it, and upload it to your main directory. Open up your notepad (start>>programs>>accessories>>notepad) or another text editor, and copy the code below to it. Name this file .htaccess and make sure it doesn't have an extension on the end, like .txt. If it is named .htaccess.txt, rename it  .htaccess. Save it and upload it to your main directory. Or, you can upload it as .htaccess.txt, and rename it when it's on your server.  If you already have a .htaccess file in your directory, because you have an IP block or something, just put the code beneath whatever you already have. Make sure you upload both 404.html and  .htaccess to your main directory. 

ErrorDocument 404 http://www.domain.com/404.html

Standart missing page