Can’t login after upgrade…fatal error

WordPress was recently moved from root to directory /wordpress and integrated within a dolphin template. I had the bright idea to upgrade to the latest wordpress version using an automatic upgrade plugin. Everything worked fine until the last step where I had to reactivate the plugins. As an error was discovered I fixed it by adding wordpress/ prior to the file in question. Up until this error…

Fatal error: Class ‘WP_Widget’ not found in /home/faithful/public_html/wordpress/wp-includes/default-widgets.php on line 15

I have gone back through the upgrade instructions manually. I’ve deleted and reuploaded the wp-login.php file. I’ve renamed the plugins directory following instructions provided. And still get the above error.

Need advice on what to try next…thank you!

Dawn