This is new to 5.2 and is called Recovery Mode. The site admin should have received an email. In prior versions of WordPress, your site would just be showing a white screen to all users, including you when you try to log in. With Recovery Mode, the site admin gets an email with a link that will allow a log in by disabling the plugin or theme with the problem.
That is the way it should work, but maybe there are some problems still to fix.
Proceed as you would if your site was just white and you couldn’t log in. (If you can log in, then the problem is on the front end only.) You can look at the server error_log to see what errors happened and fix the error.
If you can’t log in and don’t have an error_log, rename the plugins folder and you might be able to log in. But you will still not know which caused it and also have no plugins running.
