Is your website down? OMG, that is not a cause for ROFL. However, do not panic or leap to conclusions. It could be you typed it in wrong, the ISP has failed you - so other people can see it - or a host of connection issues between you and your beloved shiny webhome.
Can you reach other websites on the internet? No? Then you arn't connected to the internet. Your site is probably fine. Reboot, check the wires and call your ISP.
Can you reach other websites on the same server as your site? Yes? The server is ok. Its your site (check your code) or your domain name (check the dns)
Ping your site. Connecting? Then its your site or server - but the connection is being made.
do a whois lookup. is the domain pointing to the corect nameservers? still valid? hasn't expired?
Can you retrieve email from your domain?No? Could be server is having a problem.
do a traceroute. Complete path to the server? no? then its not the server, its the connection. Yes? then its the server.