I get a "The Page Cannot be Found" or a 404 Error when viewing my web site

This is an error produced by the web server software when it cannot find a default web page for your web site.  In some ways it's not a bad error to receive because it means the web server is working, you just need to check a couple of things out.
 

1. Are you connecting to the correct page and is there a default document?
   
   If you have just put the web sites address into your browser e.g. http://www.<thedomain.com> and you get this error, try connecting to a page you know exists.  For instance, if the home page is "index.htm" then try connecting to http://www.<thedomain.com>/index.htm. 
   
   If you are able to connect to the web site doing this, then it suggests that you need to update the "default documents" for your web site. This is a list of pages such as index.htm, default.aspx, index.php that you can tell the web server to use as the "default" document type for. 
    
2. Are you mixing different types of programmes in the same application pool?
   
   Each web site on our server runs under its own "Application Pool". If you add a subdomain to a web site, that subdomain runs under the same application pool as the web site.  If your site operates under, say, ASP.NET 4, and you create a subdomain that uses ASP.NET 3.5 then it will cause a conflict - one site may operate correctly for a while until one of the other subdomains is requested, then the error will be displayed. The same can happen when mixing PHP and ASP.NET 4 under the same application pool.