This is the message WordPress now displays when a PHP Fatal Error is thrown. Usually, the administrator of the site is being notified via email (automatically) about the problem. Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.
If your website is showing this error, then you should imediatelly check your email inbox folder, for the error details message. If the email is not on the inbox folder, also check the spam/bulk folders.
The email subject and body should look like this:
As you can see, the email also contains details about a plugin which causes the break-down problem. At the end of the email you should also see a in-dept Error Details log:
A full tutorial on how to solve The Site is Experiencing Technical Difficulties Error below: