[Matt]

Option 1: They (or their ISP) have an IPS or Firewall that has detected malicious activity from your IP (not necessarily when YOU had it) and are blocking the IP. Sadly, while it is difficult to get an ISP to give you a static IP, it can also be difficult to get a fresh one... try unplugging your cable modem or DSL modem for a while.

Option 2: Your DNS cache is corrupt, and name resolution isn't working correctly for that site. Run "IPConfig /flushdns" from a command window.

Option 3: Your firewall or other security software is in the way (someone messing with the settings in your home router possibly?). Also, your AV software could be in the way... I've seen AVG do this to sites (yahoo, google) when web content protection was enabled.

Option 4: Your ISP has decided to block that site.

Option 5: Cookies, cache... but you've already gone there.