It could be a routing problem with your ISP. Mine does that from time to time and I contact my host thinking my sites are down when in fact they aren't. Doing a traceroute will usually tell you that much. If that is the issue it is usually fixed within 24 hours on my end.
On a side note, when I want to do a remote test to check the status or lag time of a site, I use pingdom tools:
http://tools.pingdom.com/fpt/
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