I originally had my Drupal site installed in a sub-directory on my hosting account (e.g. sitename.com/portfolio), but I wanted the site to be in the root directory. So, I basically just moved all of the files from the sub-directory into the root.
This caused some issues with not being able to navigate to any pages within my site or accessing any admin link in Drupal's back end. I moved all files from my Drupal installation back into the sub-directory to see if it would fix the problem, but I'm still unable to navigate to any page other than the home page of my Drupal site.
I tried pulling down an older version of the file tree from the day before all this happened, but the problem still exists.