Can login on admin but I cant see my backend.

I just got this error on our site.
When you login to admin page it can go through and
redirect to the index page, but when you click the
very top admin navigation it wont let you go through.
it always says, ERROR: 404 NOT FOUND..
but all the normal links for pages are totally working.
i just can’t visit the admin page. please help.

here’s my site: http://www.sunna-inverters.com

im using, WordPress 3.1.3