Critical error

"There has been a critical error on this website" means PHP stopped halfway through the page. It's usually a plugin or theme update that went wrong, or a PHP version a plugin doesn't work with yet. The rest of the site and your content are fine.

How to fix it

WP Moo sees the page again on its next check, and the problem goes away by itself.

Use the recovery email

WordPress emails a recovery link to the site's admin address, with the plugin or theme that failed.

Open the link. It logs you in with the broken plugin or theme paused.
Update that plugin or theme, or switch it off until there's a fix.
Press Exit Recovery Mode at the top of the screen.

No email? Switch the plugin off by hand

Open your hosting panel's file manager and go to wp-content/plugins.
Rename the folder of the plugin you updated last, for example woocommerce to woocommerce-off. WordPress then switches it off.
Not sure which one? Rename the whole plugins folder to see whether a plugin causes it, name it back, then try them one at a time.
Still broken? Do the same with the active theme's folder in wp-content/themes.

Did the PHP version change?

Hosts sometimes move sites to a newer PHP version, and old plugins can stop working on it. Pick the previous PHP version in your hosting panel to get the site back, then update the plugin that failed.

A PHP error on the page

Instead of the friendly message, some sites show the error itself, like "Fatal error" or "Parse error". The message names the file that failed: a path with /plugins/ or /themes/ in it tells you which plugin or theme it is. Switch that one off as described above, and ask your developer or its maker for a fix. Visitors shouldn't see these messages, so also ask to turn off display_errors on the live site.

For your developer

To see the error, set this in wp-config.php:

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

The log goes to wp-content/debug.log, which anyone can download by default. Turn it off when you're done and delete the file, or the WordPress check reports it. With WP-CLI:

wp plugin deactivate plugin-name --skip-plugins --skip-themes

WordPress failure signatures lists the markup we match.

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