Jeremy said you can't see any of the sites on any of the servers. All the servers are obviously online, and there are people online in your forum. My guess would be a problems somewhere between your ISP and us. A traceroute would be the way to tell where the problem is.

Here's how to run a traceroute:
http://www.vertexhost.com/support/index.php?_a=knowledgebase&_j=questiondetails&_i=28&nav2=Network%20Related