Access Denied for All Users

I recently encountered a problem logging in. I could log in as the admin or any other user, but it would show the "Access Denied" message instead of the admin controls. The site worked fine for anonymous users, but no one could log in to make changes.

MySQL crashed and nodes display "n/a"

MySQL crashed due to a lack of hard drive space. More hard drive space was allocated and MySQL was restarted. For a moment, the site appeared to be fine. A minute later, everywhere a node was suppose to display, "n/a" appeared instead.