Fatal error after update [closed]

It looks like something went wrong during update and not all files are on the server or some of them are corrupted.

  1. Go to wordpress.org and download a package with WP.
  2. Go login to FTP of your clients site.
  3. Delete wp-includes and wp-admin directories.
  4. Upload these directories from original WP package.
  5. Upload all files in root directory of that package (index.php, wp-activate.php and so on).

After these steps everything should be fine.