Sometimes, you have to change the theme because it's not working properly anymore. Follow these steps to change your WordPress installation's theme back to the default theme.
First, go to the cPanel > File Manager, and inside of your domain's folder, go into the wp-content/themes folder and delete your active theme's folder.
Once that's done, go to cPanel > phpMyAdmin, find and open your website's database. You can find the database name in your wp-config.php file.
Click on "wp_options" to open the table. In some cases, it may have a different prefix like: "wp123_options", but the table's name always ends with "_options."
Now find "template" and "stylesheet" values, which you can find in the "option_name" column. Those fields contain the folder name of your active theme.
Double click on the name of your website's active theme to edit it. Then, type the folder name of the theme you wish to use and press enter. It will save the new name to the database. Make sure you update both the "template" and "stylesheet" fields. You may also click on the "Edit" link to edit the value. Make sure you only change the "option_value" field.
Once that's done, your website will start using a different theme.