I have faced this kind of a problem a few times. I believe it is a DNS Issue, try accessing using your IP, if that isn't the case, it may be an IPTables issue on your server, check to make sure that your IP address isn't being blocked.
Otherwise, it could be an issue with your ISP's DNS servers, call them and find out what is going on, if they say everything is working fine, tell them you want them to verify the correct information regarding your site in their DNS server.