If you're using the Devel module while developing with Drupal, you may have enabled the "Switch users" block which is great for troubleshooting and configuring permissions.

However, if you accidentally use it while your site is in maintenance mode (offline), then you may notice a problem: it won't let you back to the path "/user" to log in again!

The solution is to use phpMyAdmin or your database manager of choice to empty the sessions table. Then you'll be able to access /user without being redirected.