Step 1 - Open File Manager
The first thing you want to do is find the name of your WordPress database. To do this, navigate to File Manager, open wp-config.php file inside your public_html directory. You will find the name of the database your website is using in as a DB_NAME:
While you are at File Manager, be sure to check all the available themes you currently have on /wp-content/themes directory:
Step 2 - Open phpMyAdmin
Now, find the same database you found in wp-config.php file in MySQL databases section and open it via phpMyAdmin.
Once you are in phpMyAdmin, open wp_options table and find template and stylesheet rows, it will have your current theme name:
Just simply double click on both of the rows and edit the theme name to the one you have already installed (the name should fully match the name of the theme’s folder):
That’s it - your WordPress theme has been changed successfully!
If you wish to know how to disable plugins without access to the admin page, be sure to check this guide.