Error after upgrade to 4.7.1 "Call to undefined method CRM_Utils_System::isUserLoggedIn" (WordPress)
A few possibilities:

You may have an old version of the file customised. Check in your custom PHP folder, or do a find on the name of the file that contains CRM_Utils_System::isUserLoggedIn to see if this is a possibility. You may need to update it to match the newer version of the file, while keeping your change (a consultant will happily help you here if it gets tricky - either myself john@civifirst.com or a partner organisation https://civicrm.org/partners-contributors).

Alternatively, are your resources and URLs set correctly in your administer tab?

Failing all of those, double check the site's URL in your civicrm.settings.php file.

Good luck!