turning on debug mode
Turn debug mode on edit your wp-config.php file and find define('WP_DEBUG', false); change to define('WP_DEBUG', true); General causes of bugs 1. Dodgy plugins/templates. Remove them all and add them back one at a time. Same for themes 2. Config errors. Replace the config.php file with the template and reinstate it. Obviously copy/paste the login details for the database out before you do that, so when you recreate it, it asks for them again. 3. Code incompatibilities . You have some old code somewhere (e.g. an old php file) which was not upgraded and is now incompatible with the new php in the new issue of wordpress/plugin/template etc 4. Permission errors. chmod -R 755 /var/www/mysite chown -R www-data:www-data /var/www/mysite assuming your installation is located in /var/www/mysite. On a single-site server it will be /var/www/html "There has been a critical error on your website" = problem 3 above, usually.