After upgrading to 5.13.4 all of Civi has disappeared to be replaced with the error
Fatal error: Class 'DB_DataObject' not found in /var/sites/c/[site]/public_html/sites/all/modules/civicrm/CRM/Core/DAO.php on line 49
I first noticed it trying to visit /civicrm/upgrade?reset=1 after uploading all the updated files, but it appears trying to load any part of CiviCRM. Drupal (7.67) still loads fine, though it's still in maintenance mode at the moment.
And ideas what's gone wrong?