Fatal error: Failed opening required

Fatal error: Failed opening required ‘/data/whizardries/blogs/mickc/blog/wp-includes/wp-db.php’ (include_path=’.:/usr/share/pear’) in /data/whizardries/blogs/mickc/blog/wp-settings.php on line 30

I know what it means, but since the file exists exactly where it is supposed to, I’m not sure what I need to do in order to fix this problem.