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.

After logging into your database with PhpMyAdmin, select your particular website database (having all the content tables) in the left panel (not the schema tables).

In phpMyAdmin, select the wp_options table to open wp_options table (select the text, not the checkbox) and find the “template” row (record) and the “stylesheet” row (record). These records may not be found on the first page of the wp_options table that you are viewing in PhpMyAdmin. So, you may need to scroll down or move to the next page to find the “template” and “stylesheet” data. These rows (records) will contain your current theme name.

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

In each record, to change the theme to one that should be working, just edit and change the template name and change the stylesheet name in each of these two records to be exactly the same as the name of sub-directory of your chosen theme in your website. For example – twentytwentytwo

Note that the theme folder and files must already be installed in your content/themes/ subdirectory.

That should be it! Clear the cache in your web browser and try accessing your website from your browser. Hope this worked for you.