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Reset the URLs to their http version. You can do this via PHPMYADMIN in the wp_options table by locating wp_home and wp_siteurl and adding the http:// version of your website or by adding constants in wp-config.php:

Add these two lines to your wp-config.php, where “example.com” is the correct location of your site.

define( ‘WP_HOME’, ‘http://example.com’ );

define( ‘WP_SITEURL’, ‘http://example.com’ );

Reference: https://wordpress.org/support/article/changing-the-site-url/#edit-wp-config-php

The latter is probably the easiest.

To make your site use https, you must FIRST install a valid SSL certificate. Contact your host for assistance with this.

Credit: https://wordpress.org/support/topic/http-to-https-website-crash/