Why is my WordPress site showing a white screen or "critical error"?

The White Screen of Death and the message “There has been a critical error on this website” are both WordPress hiding a fatal PHP error. The usual causes are a bad plugin or theme update, a function that no longer exists after a PHP version bump, or a memory exhaustion. We enable WP_DEBUG_LOG, read debug.log, and trace it to the exact line, then patch or roll back the single offender.