First of all, just because you cannot ping it doesn't mean it's not up and running.
That site could be blocking ICMP traffic and thus you would get no reply.
Or perhaps it's down for maintenance.
Have you tried other protocols like HTTP?
Is the site the only one which you cannot access or are you unable to access any site on the internet?
If this site is the only one, confirm you have the correct IP address or URL and try back after several hours just incase they were performing maintenance the last time you tried.