From the Troubleshooting FAQ (http://drupal.org/node/199):
Once you have turned your site off-line using admin>settings>site maintenance, you can log back in by visiting example.com/user or example.com/?q=user and entering the username and password of an account with permission to administer site configuration. User 1 has this ability as a safety net, should you not have this turned on in other administrator account(s).