WordPress WP-Admin Access Fix

The front-end of my WordPress site loads flawlessly and I can log in without issue. However, any attempt to reach /wp-admin (backend) throws an error and locks me out with this message:

There has been a critical error on this website. Please check your site admin email inbox for instructions. If you continue to have problems, please try the support forums.

What I'm Looking For: Someone to walk me through the process to fix this so I will know what to do if it happens again.