WordPress "There has been a critical error" / white screen: triage checklist

0. Don't make it worse

Take a full backup (files and database) or a host snapshot before touching anything.
Write down what changed recently: a plugin or theme update, a PHP version change, a new snippet, a host migration.

1. Use WordPress's own signals

Recovery Mode email (WordPress 5.2+): WordPress emails the admin address a special login link that pauses the plugin or theme that broke the site. Check the admin inbox and spam folder.
Check Tools → Site Health, if the admin area still loads.

2. Get the real error message

// wp-config.php, above "That's all, stop editing!"
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );      // writes to wp-content/debug.log
define( 'WP_DEBUG_DISPLAY', false ); // keep errors off the public page
@ini_set( 'display_errors', 0 );

Reload the broken page, then read wp-content/debug.log. The "PHP Fatal error … in /wp-content/plugins/NAME/…" line usually names the culprit.
Also check the host's PHP error log in the control panel.

3. Isolate plugins and themes

# WP-CLI (SSH): test without plugins/themes loading
wp plugin list --skip-plugins --skip-themes
wp plugin deactivate NAME --skip-plugins --skip-themes
wp plugin deactivate --all --skip-plugins --skip-themes   # last resort, then re-enable one by one
wp theme activate twentytwentyfive --skip-plugins --skip-themes

No SSH? Use SFTP or the file manager: rename wp-content/plugins/NAME to NAME.off, or rename the whole plugins folder to test. Do the same for the active theme folder (WordPress falls back to a default theme if one is installed).

4. Environment causes

PHP version: compare it with the versions the plugin and theme support. A recent host upgrade or downgrade is a common trigger.
Memory: "Allowed memory size exhausted" means raising define('WP_MEMORY_LIMIT','256M'); and the host's PHP memory_limit.
Permalinks/.htaccess: if you get 404s or 500s after a move, re-save Settings → Permalinks, or restore the default WordPress .htaccess rules.
Disk full / file permissions: check the host quota, and that files are 644 and folders 755.

5. Verify core and plugin integrity

wp core verify-checksums
wp plugin verify-checksums --all   # works for plugins hosted on wordpress.org

Unexpected modified files can mean a compromise. If so, treat it as a security incident: restore clean files, rotate passwords and salts, and scan the site.

6. Fix, then roll forward safely

Update, replace or roll back the faulty plugin (for example with the wp plugin install NAME --version=X.Y --force command).
Re-enable everything one at a time, testing the home page, a page logged in, checkout or forms, and wp-admin.
Turn WP_DEBUG back off and delete debug.log (it can leak paths).
If nothing works within the agreed time, restore the last good backup and debug on a staging copy.

7. Hand-over note to the site owner

Send a short note covering what broke, the root cause, what was changed, and how to avoid it happening again (staging, auto-update rules, backups).