White screen or "critical error" on WordPress

Most often, an incompatible plugin or theme is to blame.

1. Rename the wp-content/plugins folder to plugins_off (file manager): if the website comes back, re-enable the plugins one by one.
2. Try a default theme.
3. Turn on debugging in wp-config.php: define('WP_DEBUG', true); to display the exact error.
4. Check the PHP version.