Apparently there is a server-side error causing the page to be displayed like this. Which error this is exactly, is in the error log which you should find in your hosting area. There you can also see what exactly is the cause. Often it is a plugin, which you have to deactivate to solve the problem. The support of your hoster can help you.
Since you obviously can’t get into the WP backend anymore, you would have to remove the guilty plugin via FTP. Your hosting support should be able to help you with that as well.
How to solve this error:
Warning: require(/var/www/html/wp-includes/class-wp-theme-json.php): Failed to open stream: No such file or directory in /var/www/html/wp-settings.php on line 174
Fatal error: Uncaught Error: Failed opening required ‘/var/www/html/wp-includes/class-wp-theme-json.php’ (include_path=’.:/usr/share/php’) in /var/www/html/wp-settings.php:174 Stack trace: #0 /var/www/html/wp-config.php(96): require_once() #1 /var/www/html/wp-load.php(50): require_once(‘…’) #2 /var/www/html/wp-blog-header.php(13): require_once(‘…’) #3 /var/www/html/index.php(17): require(‘…’) #4 {main} thrown in /var/www/html/wp-settings.php on line 174
