If you log out without having set up a convenient login block to let you back in... you're locked out! Right?

Fortunately, there's an easy way back in: <your domain>/user will always provide you a login form. (Make that <your domain>/?q=user if you don't have "clean URLs enabled.)

Yep, I've done the Locked-Out Panic. So have countless others; it's THE emergency de rigeur among Drupal initiates. Get a Drupal tattoo to mark the rite of passage, then log yourself back in!

Additional resources

How to login once you have turned your site off-line for maintenance