Why this happens

PHP is trying to create an object from a class definition that doesn't exist in the currently loaded files. This usually means a plugin's files were only partially uploaded or extracted, a required add-on isn't installed, or the plugin was updated in a way that removed a class another part of your site still references.

Step-by-step fix

  1. Check the error for the plugin name and uninstall that plugin completely.
  2. Download a fresh copy from WordPress.org or wherever you originally got it.
  3. Reinstall and activate it.
  4. If the issue persists, contact the plugin developer - it may not support your current WordPress version.

How to prevent it happening again