Just tried it out, and everything seams to be up and running. For future references, you may want to check sites on
http://www.downforeveryoneorjustme.com/If they were indeed down, and still you can't load the site, it's possible that you need to wait for their DNS to refresh.
That is a great tool too in real time, but from the link that i provided seems that they were offline past 24 hours:
"Our team has taken action to mitigate a possible DDoS attack on the Coinbase.com website. Service has been returned to normal, the site is online and fully operational."