That likely means the update broke and messed up your WordPress installation. But there is no need to worry, you can still recover the installation.
To fix this, you can download a fresh copy of WordPress of the specific version you were upgrading to: Download – WordPress.org
Then, you extract the archive on your computer and upload it’s contents on top of your existing website, overwriting any existing files.
That should ensure all the WordPress core files are present and compatible, and allow WordPress to fix itself from there.