There has been a critical error on this website.

Or

There has been a critical error on this website. Please check your site admin email inbox for instructions.

Do you have “There has been a critical error on this website.” on your WordPress website.

Check in the root folder of your WordPress installation whether there is a “.maintenance” file.
The presence of this file confirms the WordPress upgrade/update was not cleanly finalized.
Usually the files looks something like this:

<?php $upgrading = 1408322315; ?>

Sometimes just remove that file, is enough.

But often it means the update process got interrupted (by network, memory issue) causing your WordPress having code files from version XYZ ánd code files from version ABC.

The next thing to check are the log files.

Do you see something similar as this:

Stack trace:
#0 /home/stijn/domains/mydomain.be/public_html/wp/wp-config.php(90): require_once()
#1 /home/stijn/domains/mydomain.be/public_html/wp/wp-load.php(37): require_once('/home/stijn/...')
#2 /home/stijn/domains/mydomain.be/public_html/wp/wp-blog-header.php(13): require_once('/home/stijn/...')
#3 /home/stijn/domains/mydomain.be/public_html/wp/index.php(17): require('/home/stijn/...')
#4 {main}
  thrown in /home/stijn/domains/mydomain.be/public_html/wp/wp-settings.php on line 70
2021-02-03 00:49:59.024474 [NOTICE] [990825] [45.158.15.36:53882#APVH_www.mydomain.be] [STDERR] PHP Fatal error:  Uncaught Error: Call to undefined function wp_unregister_GLOBALS() in /home/stijn/domains/mydomain.be/public_html/wp/wp-settings.php:70

If you have a recent backup, probably the easiest thing to do is to restore that back up.

Make sure you have the backup on your local disk.

Login into your Hosting Account (Cpanel, DirectAdmin or …).
Delete the unstable WordPress website.

Install a fresh WordPress installation.
Restore your website using the backup you save on your local disk.

Or do this:
https://wordpress.org/support/article/updating-wordpress/#step-1-replace-wordpress-files
Tip: how to know which plugins are active, you can use this query:

SELECT * FROM wp_options WHERE option_name = 'active_plugins';