When someone asks you a phone number, you look it up in the phone book. If someone else asks for the same number, you don't bother looking it up, because you already have it in your head.
Computers do the same thing. Your ISP has to pay a charge each time it moves data across the lines it uses. So, if an ISP's customers try to load the same page twice, they can save a few pennies by storing that page on their own computers, and sending it to you the second time, without having to use their external lines. And that few pennies is multiplied by millions and billions of accesses.
But, as with other aspects of computers, sometimes this scheme fails.
When your ISP's cache fails, the effect you get is that there's some web sites that you just can't access, even though there's nothing wrong with the web sites. You can check that, by accessing via a different ISP (but obvously that mustn't be the same ISP under a different name, which is reasonably common, because of all the mergers).
Do a full browser refresh (Ctrl-Refresh in IE, or Shift-Reload in Netscape) twice, with the second one following the first after a gap of at least 1 and not more than 10 seconds. That's easy to try. However, in my experience, it doesn't work.
Here's a method that I've tried, and it does work. But it can be very slow surfing.
Another possibility is www.the-cloak.com
If it doesn't work, go to Silentsurf and surf via their site.
If you can access via Silentsurf, then that's a workaround. Also, you can then report the problem to your ISP, and if they reply that this means that the web site must be down, you can tell them that you know it isn't, because you can access via Silentsurf. Also, tell them that this is a problem for their network administrators, it isn't something that the front line help desk will be able to fix. So they'll need to escalate the problem.
Here's a more detailed and technical explanation of the problem, you might give that URL to your ISP to help them sort it out.