You updated, the site went down, and the admin panel won’t open either. This is recoverable, and the path doesn’t go through the panel: it goes through FTP or your hosting’s file manager. Before anything else, take a breath: your data is in the database, intact. What broke are files.
First: find out what broke
If the screen is white, with no message at all, WordPress is hiding the error. To see it, edit wp-config.php at the root and change the WP_DEBUG line to define('WP_DEBUG', true);. Reload: now the file and line that failed show up, and that usually names the culprit right away.
Once you’re done diagnosing, switch it back to false. A visible PHP error on a public site is information people shouldn’t have.
If it was a plugin
Via FTP, go to wp-content/plugins/ and rename the suspect plugin’s folder, from plugin-name to plugin-name-off. WordPress can no longer find the main file and deactivates the plugin on its own, without deleting any data.
If you don’t know which one it was, rename the entire plugins folder to plugins-off. All of them deactivate at once and the panel comes back. Then rename it back and reactivate them one by one until you find the culprit.
If it was the theme
In wp-content/themes/, rename the active theme’s folder. WordPress automatically falls back to a default theme: ugly, but up, and with the admin panel accessible.
If there’s no default theme installed, download one from WordPress.org and upload the folder there.
Roll back to the previous version
With the site back up, reinstall the version that worked. If you kept the old ZIP, that’s the one. If you didn’t, this item’s page here on the site has a previous versions table, with versions and dates.
Users of the Ultrapack Auto Updater have this path shortened: it takes a copy (snapshot) of the item before every update and lets you restore the last working state right from its own panel.
Once it’s back up
It’s worth figuring out why it broke before trying again. The most common causes are a PHP version below what the new version requires, a plugin conflicting with another one, and customizations made directly in the theme folder, which the update wiped out. That last one has a definitive prevention in a child theme.
And for next time: a staging environment turns this whole episode into five minutes with no audience.
