charging zen, problem

i bought some pre-paid cards today and typed in (billing.perfectworl.com) as instructed in the back on the cards. when i typed in the address, it told me that the web page is unavailable. My internet is running fine and other sites are ok too, like youtube and PWI homepage, but this zen charging page was the only one that was not working. Is something going on or wrong with that page? is it only my computer? this happened on july 18th, 10am eastern time.
thank you