Edit functions.php
If you have access to the site via FTP, then this method will help you quickly get a site back up and running, if you changed those values incorrectly.
1. FTP to the site, and get a copy of the active theme's functions.php file. You're going to edit it in a simple text editor and upload it back to the site.
2. Add these two lines to the file, immediately after the initial "<?php" line.
update_option( 'siteurl', 'http://example.com' ); update_option( 'home', 'http://example.com' );
Use your own URL instead of example.com, obviously.
3. Upload the file back to your site, in the same location. FileZilla offers a handy "edit file" function to do all of the above rapidly; if you can use that, do so.
4. Load the login or admin page a couple of times. The site should come back up.