Error Message

What did you update from and to? How did you do the update? I’d guess that something went amiss during the update and I’d try do a manual re-install of the core files.

Try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.