The Underscore Archives

This is the help text from the maintenance mode page:

When set to "Online", all visitors will be able to browse your site
normally. When set to "Off-line", only users with the "administer site
configuration" permission will be able to access your site to perform
maintenance; all other visitors will see the site off-line message
configured below. Authorized users can log in during "Off-line" mode
directly via the user login page.

...so you should be able to login if you go to yoursite.com/user (or
?q=user if you've not got clean URLs).

cheers,
Drew