I had this problem yesterday, and it took a lot of research to figure out this fix. I notice there wasn't much information on this topic, so here it goes. Hope someone else profits from it.
My mistake was to change the 'wordpress address' (URL) and 'Blog address' (URL) under general settings, I changed it from http://editnetworksupport.com to http://editnetworksupport.com/blog; I saved the changes, and visited my site to find only content on my site with a blank background, the website theme had disappeared. No problem right, go back and retrack the changes? Well, I wasn't able to login to wp-admin. DON'T PANIC!
here's your fix
BACK UP YOUR FILES FIRST!!!
With an FTP software, i use FileZilla, login to your host server, all of your files should still be there, i thought mine would be gone, but they weren't. Go to your active themes and locate 'functions.php' file, and edit it with notepad; you will input the following, on the very top part after <?php
Save your changes, and try accessing wp-admin, you will have to try a few times before it works, I had to wait a few minutes.
As for my web theme, I was missing my index.php file, luckily I had a backup and I was able to restore my files.