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

What it means

WordPress stopped because some PHP code crashed. Since WordPress 5.2, instead of showing the technical error to visitors, it shows this short message and emails the site admin more detail. Your content is almost always still there. The site just cannot finish loading the page.

What usually causes it

A plugin or theme update that is not compatible with your WordPress version, another plugin, or your server’s PHP version. This is the most common cause.
Your host upgraded PHP (for example from 7.4 to 8.x) and an old plugin or theme uses code that newer PHP no longer accepts.
Running out of memory on a busy page, often with page builders or large WooCommerce stores.
A half-finished update, for example if the upload was interrupted.

What you can check yourself (about 5 minutes)

Look for the email. WordPress sends a message to the admin address with the subject “Your Site is Experiencing a Technical Issue”. It usually names the plugin or theme that crashed and contains a special recovery mode link.
Use the recovery mode link. It lets you log in even though the site is broken. WordPress pauses the faulty plugin for you, so you can deactivate or update it from the Plugins page.
Think about what changed. Did you, or automatic updates, update something in the last day? That plugin is the first suspect.
Check your host’s dashboard. Many hosts show a notice when they change the PHP version, and some let you switch back temporarily.

If you have no email and no recovery link, do not start deleting folders on the server. Renaming the wrong folder can take down more than you intended.

When to ask for help

The email never arrived, or the link does not work.
Deactivating the plugin fixes the error but breaks something you need, such as your shop or booking form.
The error comes back after every update.

Send me the text of the WordPress email, if you have it, and tell me what changed recently. I can usually find the cause from that alone.