I have put a site in maintenance mode. Before I could visit the administration pages as user #1 (which was the user account I was already using before to put the site in maintenance mode), I had to reset the computer; when I tried to visit the administration pages, Drupal didn't give me access to those pages.
I cannot change the content of the settings.php file because the installer tool that I used from the control panel for the domain probably created the files using credentials different from the ones I use to connect through FTP, which is the only way I have to upload files to the server, or change them.
Apart re-installing the Drupal files manually (and in a different directory), what can I do to access the administrative pages?
You should be able to log in by going to http://example.com/user directly (or http://example.com/index.php?q=user if clean URLS are turned off).