If a site has nameserver issues your machine, or even your ISP, may not refresh the cache for hours or days. A quick way to check is just to visit a site via a free online proxy. A lot of the time you'll see the site up (so you know it's ok) but you still see it down locally. On a few occasions I've had to actually phone my ISP and get them to flush their cache because the whole island sees a site down when it is up worldwide.