Most likely cause (by far):
The website is down (either temporarily or permanently).
Solution:
None, except that you can check by getting someone else to see if they can access the site from their computer.
Possible cause:
The website doesn't like your browser for some reason.
Solution:
Try a different browser.
Another possible cause:
The website doesn't like your IP address. (That might be because it's a dynamic address that has previously been associated with hacking).
Solution (assuming that your ISP uses dynamic IP addresses, as most do, and not static ones):
Go here to check your current IP address:
http://whatismyipaddress.com/
Disconnect your router from your phone line, wait 30 seconds and then reconnect it. Go to the link again to confirm that you've been allocated a different IP address and then try to access the troublesome site again.
Yet another possible cause:
Your security software is blocking the site.
Solution:
Look through the logs of your security software to see if there's any record of the site being blocked. If so, look for an option to declare the site safe. Alternatively, temporarily disable your security software to see if you can then access the site.