I just attempted to upgrade to WordPress 3.4.2 received a "Missing a critical file" failure. The error was
"Refresh Failed: status:198|file:/home1/website/public_html/wp-includes/version.php"
I believe the problem was that I recently moved WordPress from my root directory to a /blog sub-directory following the Codex instructions. It seemed successful, but something was not updated and the upgrade tried to work in the wrong place.
Now I cannot gain access to to my WP dashboard, and most of my blog contents is missing.
The only solution I can think of is to uninstall and re-install wordpress; would that work? Is their possibly an easier fix?