I just upgraded from CiviCRM 4.7.10 to 4.7.24 and I followed the steps on the update page here. When I copied the new version to the plugins folder, I started receiving this error in the admin portion of WordPress when I tried to load any pages of CiviCRM.
[20-Sep-2017 20:49:48 UTC] PHP Fatal error: Uncaught Error: Class 'CRM_Extension_ClassLoader' not found in /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm/CRM/Extension/System.php:196 Stack trace: #0 /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm/Civi/Core/Container.php(395): CRM_Extension_System->getClassLoader() #1 /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Config.php(112): Civi\Core\Container::boot(true) #2 /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm.php(672): CRM_Core_Config::singleton() #3 /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm.php(1121): CiviCRM_For_WordPress->initialize() #4 /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm.php(969): CiviCRM_For_WordPress->add_core_resources(false) #5 /home/prairie1/public_html/wp-includes/class-wp-hook.php(298): CiviCRM_For_WordPress->admin_page_load('') #6 /home/prairie1/public_html/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array) #7 /home/prairie1/public_html/wp-includes/plugin.ph in /home/prairie1/public_html/wp-content/plugins/civicrm/civicrm/CRM/Extension/System.php on line 196
I made sure that civicrm.settings.php had the following at the end:
require_once 'CRM/Core/ClassLoader.php'; CRM_Core_ClassLoader::singleton()->register();
I also started off with php version 5.5 running, but I read that I could try 7 and it was supported, so I switched. It gave me the stack trace where 5.5 did not, but didn't help the error.
Thanks a lot!