Changing the URL where your WordPress install is living at isn't a difficult process, however, it's easy to get tripped up in a few areas, especially because WordPress has a setting under 'Settings' > 'General' that makes you think simply changing the field here should work and instead it will break your website.
Fix: In this case the user wanted to get /blog off the end of the URL but by changing the address here and was then locked out of the site. To return the site back to normal, go to 'Databases' > 'phpMyAdmin' in your cPanel:
You should see a list of your databases on the left. Expand the WordPress database for your install (if you only have one site there will be just this one) and choose wp_options as the table to edit.
The wp_options table holds two fields that correspond to the fields you saw earlier in the WordPress settings, siteurl and home. Both will need to be changed back to what they were previously for the site to load at the old URL.
siteurl should be on the first page and often home is on the second page. You can click Edit next to the field and update the URL to what it was previously and then click Go to save.
After editing those two fields the site should come back online at the old URL.
Now you can get back to changing your URL correctly by moving the WordPress install to the new location. The easiest way to do this is to view our guide on cloning your install to a new address.
If you want to buy a new domain first and add it to your account to move the install to that new domain our Additional Domains guide can assist you with that process.