On the off chance that anybody else was using this site and saw the offline maintenance message for the ten minutes or so that it was up, I upgraded Drupal since the latest release fixes a couple of security holes.

Fun story: I accidentally locked myself out of the site after upgrading it. You can't call AAA for that. To get the site out of maintenance mode, you have to go into MySQL and turn off the site_offline Drupal variable:

update variable set value='s:1:"0"' where name = 'site_offline';
delete from cache where cid='variables';

's:1:"0"' is how Drupal stores its variables as strings; that's not SQL syntax, in case anyone was wondering. The second line clears the variable cache or else Drupal is still going to think the variable is set.