Do you use a fully dynamic IP? Perhaps whomever it was assigned to last time abused it, prompting them to temporarily block or ban it? Not sure what kind of error code the site would spew out in that circumstance, or if it would allow the IP access to its domain in the first place. You could test this by checking if it works now, writing down your current IP, and if it stops working again in the future, check to see if you have a different address and if resetting your router and swapping IPs would make the difference.
