There has been a critical error on your website. Please check your site admin em

check your spam folder. These emails often get spammed.

Errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.

Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://www.ads-software.com/support/article/debugging-in-wordpress/

You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you cannot access wp-admin, there are other ways to deactivate plugins.-

In 5.3 the admin email gets checked from time to time to make sure the admin email address set is proper and working.

So you need to make sure the admin email address is right and the email function of your host works.

If email doesn’t work then I’ll refer you to this article to setup email vial SMTP instead. That’s not super easy though so ask your host about the email issue first if you have trouble with it.

https://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-not-sending-email-issue/

As to your site having issues. Most of those will be listed in your error logs and health check may help you find some info on them also.

Make sure your PHP is updated. You’ll want PHP 7.3 and PHP 7.4 is supposed to be released on the 28th of this month. WordPress 5.3 should support either of those versions. Version 7.2 should also work just fine.

Your web host should be able to help you with those changes.

Hope this helps. Let us know if you need anything else.