I had a client contact me this morning that their site had crashed. They received an email message from their host provider saying their version of WordPress had been upgraded. I saw the error message and most likely the upgrade failed, hosing the site. This can only be fixed via FTP. Even the login screen gives the same error message.
I won’t know until I have access to the client’s server to determine what the actual problem is. It could also be a plugin or theme conflict.
Anyway, before warning or advising my other clients about upgrading to the just released WordPress 4.5 I thought I’d go ahead and upgrade this website. I backed up all my files and database and then I updated all my plugins and themes. I then ran the WordPress update and it updated successfully.
It does happen that updates fail and it could be WordPress, a theme or a plugin. You can manually deactivate themes and plugins via ftp access by renaming their respective folder. This breaks the connection and sometimes the error goes away. If it’s not a plugin or theme then WordPress is hosed and you’ll need to manually reinstall the files via FTP.
Update: I checked their site and it was missing files from the WordPress install. I replaced them and their site was happy again.