If you moved the folder of the root of the site, so that the URL is https://www.example.com instead of https://www.example.com/blog , then you need to tell the database that’s where the site lives.
This is done by modifying two entries in the wp-options table to change the URL. You will also need to change the URLs that are stored in the database for media files – each media file has it’s full URL as part of the database.
To do that, you can use a “search and replace” plugin (I like “Better Search and Replace”, but there are others) to search for the old URL (https://www.example.com/blog ) and replace it with the new URL (https://www.example.com ). But that should be done after you can get to the site and the site’s admin area.
You may also need to change your htaccess file, since you moved the site’s URL out of a folder to the root of your domain.
There are several help files you might find useful:
- how to move your WordPress https://codex.wordpress.org/Moving_WordPress
- your htaccess file: https://codex.wordpress.org/htaccess
- database config entries in wp-config.php : https://codex.wordpress.org/Editing_wp-config.php