ERROR Upgrading to CiviCRM 4.7.28 - Warning: require_once(CRM/Core/ClassLoader.php):
Check if the file ClassLoader.php is on the file system. The location for Joomla is:

../administrator/components/com_ci‌vicrm/civicrm/CRM/Co‌​re/ClassLoader.php


Part of an upgrade procedure is to remove the old version and unpack the new version in the same place. If the unpacking fails the above error is shown.

Another reason for the same error can be the wrong value of $civicrm_root in the civicrm.settings.php file. More information in question Fails to open ClassLoader.php file .