The website encountered an unexpected error.

https://www.drupal.org/forum/support/post-installation/2015-01-30/how-to-edit-settingsphp

After updating Drupal Core if you encountered error displayed in image above make sure you have cleared the cache.

This is also possible that you can’t access the /admin/config therefore access performance link directly at this url /admin/config/development/performance

In my case I upgraded Drupal Core and got The website encountered an unexpected error upon accessing the /admin/config.

In order to understand the cause of this issue, I added following line of code to the sites/default/settings.php

$config['system.logging']['error_level']='verbose';

And issue was resolved by simply clearing the cache, in your case error could be different.

Author: Danyal
I'm working as programmer since 2002. Born, grew up, studied and worked in Pakistan now Im located in Denmark since 2006.