1. Check your email
When a plugin or theme causes a serious error, WordPress writes to the site's admin email address. Its message names the plugin or theme at fault, and gives a link to “Recovery Mode”: it opens the admin area despite the error. Deactivate whatever is causing the problem there. This link doesn't last: use it quickly.
2. Deactivate the plugin without the admin area
No email? Open your site's files, by FTP or with your host's file manager. In the wp-content/pluginsfolder, rename the folder of the plugin at fault: WordPress deactivates it.
If you don't know which one, rename the whole plugins folder, then open the admin area: WordPress deactivates all plugins. Then put the folder's name back, and reactivate the plugins one by one: the one that brings the site down again is the culprit.
3. Check what doesn't depend on WordPress
If every page shows an error, or the browser can't even find the site, check that:
- your domain name hasn't expired;
- your hosting is paid, and your host isn't reporting an outage;
- your site's https certificate is still valid.
4. Restore a backup
If none of this is enough, restore the last backup from before the outage. Without access to WordPress, a backup plugin installed in the site often can't do anything any more: go through your host, or a tool designed to work without WordPress.
Once the site is back, look for the cause before redoing the update that broke it.