On shared hosting, the web server must match the hostname provided by your browser to a virtual host being served at the IP address the hostname resolves to. Think of it as looking up someone's number in a telephone book, dialing the number and asking to speak to that person because you can't assume that only that person will answer the phone.
So first, are you trying to access the site using http://n.n.n.n/ or http://www.yourdomain.com/ ?
The first is always going to fail unless the web service for the site has been assigned to a dedicated IP address.
Otherwise, generally its just a matter of time for the web service or DNS to sort themselves out and things are working again. Though you might want to check with support.
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