I manage several WordPress websites, and successfully recently used the Automatic Upgrade feature to bring them up to 3.1. However, on one of the sites, I received a 503 (I believe it was 503). After that I was presented with "The service is unavailable." whenever I tried to access any page, including the index page, wp-admin, etc. I had similar problems before when a WordPress site got stuck in maintenance mode, and all I needed to do was login via FTP and rename or delete the .maintenance file. I tried that in this case, but that didn't do the trick. I am now presented with "Fatal error: Call to undefined function require_wp_db() in **\wp-settings.php on line 71"

I can't figure out how to fix this problem, and I was wondering if anyone else had any ideas.

Any suggestions are appreciated! Thank you for reading.