When your Wordpress installation shows a critical error then the issue is always with your plugins or theme. Sometimes this error goes away if you refresh the page, but sometimes it doesn't. In that case, you have to enable WP_DEBUG to see what's wrong. 


Click here to see how to enable WP_DEBUG


Sometimes you may be unable to log in to wp-admin to update your plugin(s) then you have to disable the conflicting plugin.


To do that go to cPanel > File Manager, find your domain from the public_html folder and go to wp-content > plugins and just rename your plugin's folder that's causing trouble. Then visit your website again and see if there are any more errors. Most of the time your website's errors are gone and you are able to log in to wp-admin and the critical error is gone.