“There has been a critical error on this website.”

“There has been a critical error on this website.” means code in a plugin, theme or WP itself hit an error it can't get past, so WP stopped.

• A plugin update with a bug, or one that clashes with another plugin or your theme.
• Your host moved to a newer PHP version (the language WP runs on; your host sets its version), and an older plugin or theme can't run on it.
• A code snippet pasted into your theme's functions.php file, with a typo or a missing bracket.
• Your site ran out of memory. The error log says “Allowed memory size … exhausted”.
• An update that stopped halfway and left a plugin's files missing or half-copied.

What you see

Visitors see one line, “There has been a critical error on this website.”, and a link to WP's help pages. Logged in, you see more: “Please check your site admin email inbox for instructions.”

It often breaks every page, your dashboard too (the screens you log in to at yoursite.com/wp-admin). Sometimes only one page breaks, because only that page runs the broken code.

Your posts and pages are still in your database. WP stops before it can show them.

How to find the cause

The recovery-mode email

WP emails the Administration Email Address set in Settings › General. The subject ends in “Your Site is Experiencing a Technical Issue”. It names the plugin or theme, and the file and line that failed.

Its link logs you in to recovery mode. There, the broken plugin is paused for you only, so you can switch it off. The link runs out after a day.

WP sends at most one of these emails a day, and some hosts can't send mail at all. Look in your spam folder. Still nothing? Use the error log.

The error log

You can ask WP to write every error to a file. Open wp-config.php in your site's main folder with the file manager. Above the line with “That's all, stop editing!”, add these 3 lines:

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

The last one keeps errors off your pages. Reload the broken page. Then open wp-content/debug.log and find the last line that starts with “PHP Fatal error”.

That line ends with a file path. If it has wp-content/plugins/ in it, the next folder name is the plugin that crashed. If it has wp-content/themes/, it's your theme.

Many hosts also keep an error log in their control panel. When you're done, set WP_DEBUG back to false. Anyone who guesses the log's address can read it.

How to fix each cause by hand

A plugin

Switch it off in recovery mode, or rename its folder in wp-content/plugins, say from contact-form to contact-form-off. WP switches off a plugin it can't find.

Then look at the plugin's page in the WP plugin directory for a newer version or a known problem. If an update caused it, go back to the version that worked. Or leave it off until its makers fix it.

Can't tell which plugin? Rename the whole plugins folder to plugins-off. If the site loads, open Plugins › Installed Plugins once, so WP marks them all as off.

Then rename the folder back to plugins. Switch them on one at a time, and reload your site after each, until it breaks again.

Your theme

If the path points to wp-content/themes/, rename your active theme's folder. Then log in and open Appearance › Themes. WP switches to its default theme, if that's installed.

A snippet pasted into the theme's functions.php file is a common cause. Remove it with the file manager. If it was the cause, the site comes back.

A PHP version change

Did the crash start when your host changed your PHP version? Then an older plugin or theme may not run on the new one. Update it, and if there's no update, ask your host about the version.

Not enough memory

A log line with “Allowed memory size … exhausted” means WP ran out of memory. Add define( 'WP_MEMORY_LIMIT', '256M' ); to wp-config.php, above the same line. Your host may set a lower cap, so ask them if it doesn't help.

A half-finished update

An update that stopped halfway can leave a plugin's files missing. Delete that plugin's folder with the file manager, then install a fresh copy from the WP plugin directory. Its settings stay in your database.

How to stop it coming back

Update one plugin at a time, and open your site after each one. If it breaks, you know which update did it.

Keep the Administration Email Address in Settings › General on an inbox you read. That's where WP sends the recovery-mode email.

Try new code on a test copy of your site first. Many hosts can make one for you. Before big updates, ask your host how you'd put back an earlier copy of your site.