One other thing to generally try: start->run ipconfig /flushdns

If a provider changes their dns settings (like moving to a different server when under attack), your PC will "remember" the old address for however long was specified by the provider before the change. Often, these are hours if not days. Additionally, when a site is down, Windows "remembers" not finding the site for some period of time as well.

Clearing DNS cache can solve a whole bunch of name resolution issues.

Note; sometimes your ISP will also be caching ... you can override this by using a different DNS provider - or at least using something like opendns to check vs. the address you see.

Last suggestion for future use - also check with netcraft (netcraft.com). They have a tool that can be used to check to see whether the site is up from their perspective. If they see the site up and you don't, then you can start troubleshooting. If they also see it down, it's probably down.