It could be that you are behind a proxy that is unable to connect to your website (in that case, you may see the name of a proxy application somewhere on the page); or it could be that your website somehow can't be served to you, for whatever reason.
If it really is a server error and not a Proxy error, you would have to consult your server's error log files to find out the reason for the problem. You will usually find a detailed explanation of the problem there.
