Some Admin disappeared

I’d look at your error.log file just after you access the admin page. I’d suspect that there is a plugin code that is failing on the admin page. The error log might point you to the culprit.

Alternately, you could disable recently added/updated plugins by renaming that plugin’s folder (in wp-content/plugins folder), which will effectively disable it. That will disable that plugin’s features, but you can quickly re-enable by changing the plugin’s folder name back to the original name.

It’s possible that some malware is on your site, but I’d suspect a bad plugin first. You can manually look at files via your hosting’s File Manager or FTP. Don’t forget to look at the wp-settings.php and wp-config.php files — I have seen malware write obfuscated code in those files.