Fatal error: Call to undefined function wp_cache_get after update attempt

I think W3 Total Cache or any Cache related plugin is still activate in your site.

Try to de-active all plugins manually.

Using Database

Open Phpmyadmin Database of the website.

1. In the table wp_options, under the option_name column (field) find the active_plugins row.
2. Change the option_value field to: a:0:{}

Using FTP or SSH

In case you don’t have access to cPanel you can do this through FTP.

1. Login in FTP and open to wp-content
2. rename plugins folder to plugins_deactive.

This both tricks will disable all plugins.

After testing, Don’t forget to rename plugin’s folder name to plugins.

If still not works, Then it can be theme issue.

Hope this help!