"There has been a critical error on this website" (WordPress)

WordPress shows this when some PHP code crashed. Almost always it is one plugin or the theme, often right after an update.

What to do, in order

1. Check your admin email. WordPress usually sends a message naming the plugin or theme that crashed, with a recovery mode link. Use the link to log in and deactivate it.
2. No email? Disable plugins by hand. Over FTP or your host's file manager, rename wp-content/plugins to plugins-off. If the site comes back, rename it back and disable plugins one at a time to find the bad one.
3. Switch the theme. Rename your theme's folder in wp-content/themes. WordPress falls back to a default theme, if one is installed.
4. Read the error. Add define('WP_DEBUG', true); define('WP_DEBUG_LOG', true); to wp-config.php. The real message lands in wp-content/debug.log. Turn it off again afterwards.
5. Check the PHP version. A host that upgraded PHP can break an old plugin. Your host's panel lets you switch versions back while you update.

If it still fails, send me the address and I will tell you which step is broken.

Want the rest of your site checked after it is back? Run a free 40-point check at tinkeraudit.com.