Have you ever experienced an outage of your Internet connection? Has your router been confused? Sometimes the Top Sites wall gets pretty corrupted, the site previews are all showing errors - or they just seem to be a bit outdated. There is a very Apple-like Solution.
After you have fixed your network-related issues, just go to your Top Sites and hit Command+R (or click on the reload arrow in the right of the URL bar).
[crarko adds: Yes, it's kind of obvious but sometimes the obvious things are the ones we miss. I'll put this up with the likelihood that it will help somebody out.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110304164334446