My client reported an issue where all of a sudden they were not able to reach the admin section of the site. Sure enough, I tested and had the same result. I went to http://www.domain.com/admin and got a Silverstripe 404 page. The site is running version 2.4.0 and there has been no changes to the extensions or core since we launched it. I searched the forum and found this related entry: http://www.silverstripe.org/general-questions/show/273695. However, the temp directory permissions were fine. Anyone had a similar issue?
Thanks for the suggestions, Willr. After wracking my brain trying to find the cause, I found that the client had accidentally deleted the 'cms' folder. This only became apparent when I attempted to upgrade from 2.4.0 -> 2.4.1 to try to resolve the issue. I probably should've checked there first!