You need to get access to the database ( and for this you need cpanel access probably ) – commonly, this is via PHPMYADMIN, most hosts have it and then you need to go to the wp_options table and find two references, these are normally on the first and second page of rows – they are:
site_url & home_url
Both should show the new URL you changed the site to – you can edit them to what you want.
As a work around if you have working FTP access, these values can also be hard coded into the wp-config.php file, in the root of the WordPress install – using the two following CONSTANTS:
define( 'WP_HOME', 'http://example.com' );
define( 'WP_SITEURL', 'http://example.com' );
Read more about how here: https://wordpress.org/support/article/changing-the-site-url/#edit-wp-config-php