First, find out what changed
Note what has just updated: the versions on the Plugins page, and the email WordPress sends after an automatic update, will tell you. If the admin area still opens, deactivate the plugin at fault: often, the site comes back. If it no longer opens, see Your WordPress site won't load.
Getting back to how things were
Deactivating isn't always enough: when it updates, a plugin can change its data in the database. In that case, restore the backup made just before the update, not an older one: you would lose more.
That's why you need one before every update, automatic ones included.
Stopping it happening again
- One update at a time. If the site breaks, you know which one did it.
- Turn off automatic updates for the plugin that broke your site, until a fixed version comes out.
- Take your time with a plugin your site depends on (shop, bookings): read what changes before updating.
- Try it on a test site first, if your host offers one.