From where they disappeared? If you create a new tag, is the menu for tag groups empty?
Please deactivate other plugins and switch to a default theme and then reload the Tag Groups page. If the page works now, enable the plugins one-by-one and see where the problem occurs.
You can additionally try the following:
In your wp-config.php file, set:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
then reload the page that caused the problem and check the debug.log file inside the /wp-content/ directory for messages that occurred while loading the page.
See more info here: https://chattymango.freshdesk.com/solution/articles/5000750260-i-only-see-a-spinning-wheel-on-the-tag-group-page