How to fix “There has been a critical error on this website” in WordPress

A printed log strip with one line highlighted in orange, next to the headline "Read the first line"

Your site shows one sentence on a white page, “There has been a critical error on this website”, and nothing else loads, often not even the login screen. The message hides the real problem on purpose, but WordPress knows exactly which file broke and tells you in two places: an email to the site admin […]