Lost Rich Text Widget

Have you tried:
– Troubleshooting with your browser. Your browser can help you identify JavaScript issues or conflicts and this article can assist you in doing that diagnosis. This could help identify Visual Editor issues as well.
– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– switching to the unedited default Theme (Twenty Seventeen.) for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins).
– MANUALLY updating. Download a fresh copy of the WordPress .zip file to your computer, unzip it, and use that to copy up all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory. You may need to delete the old wp-admin and wp-includes folders and files on your server before uploading the new ones. Please read the Manual Update directions first.
– Backup: If you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. You can never have enough backups!