Critical Error

“I just upgraded my Premium plan to a Business plan ” This sounds like you’re using wordpress.com. If so, due to the differences between the various site hosting plans offered at wordpress.com and the self-hosted and self-installed version of WordPress supported here, you should probably ask WordPress.com support instead: https://wordpress.com/support/help-support-options/

If it’s a self-hosted site at another host, errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.

Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://wordpress.org/support/article/debugging-in-wordpress/

You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you cannot access wp-admin, there are other ways to deactivate plugins:  https://wordpress.org/support/article/faq-troubleshooting/#how-to-deactivate-all-plugins-when-not-able-to-access-the-administrative-menus