If you activated a different theme or experimental child theme, and your site went down or errors don't permit you to login with administrative credentials, you should still be able to change the current theme through phpMyAdmin. You may want to merely revert to the theme that was working before you activated a different theme.

Go to your phpMyadmin in your web browser and log into your website's database with the username and password credentials that has/have permissions to access your MySQL or Maria database for you website.

Select the database in the left panel (not the schema).

On the right, select the Options table (the text, not the checkbox)

Use the > to go to the next screen to locate and edit the “template” and “stylesheet” records.

In each case, change the template name and change the stylesheet name to be exactly the same as the name of sub-directory of your theme in your website. For example – twentytwentyone