Curcuit breaker not accesible
Sometimes the breaker boxes will have a large Main Disconnect switch separated from the others.  If so, it should disconnect all power, including to your secondary breaker box.

Even if you have a main disconnect, you can never be sure how it was wired.  So, if you do have one, use your meter to make sure that the feeds to the secondary breaker box are, in fact, unenergized, before you start working on it!

Good luck :)