Google Adsense can go down for many reasons, most of which will be related to one of the hops or machines between you and Google's adsense datacenter being in the middle of an update of its DNS for Google Adsense. This usually won't last more than an hour or two. The other reason is that Google is itself doing an update or repair to their datacenter for your area. You can know how this happens by seeing the actual notice from Google that the system is 'unavailable' when you try to login to the Adsense page.
Ok, they are back up. Thanks to all of you for your responses. I appreciate it. I was beginning to worry about my account.
It works fine, but in general google had many problems latelly. I had problems with adsense and gmail.
I had a load of irrelevant ads in Japanese with nothing but question marks as the subject. I have had 3 clicks though so I can't complain too much.