You should troubleshoot your website for plugins and WordPress Themes.
Make sure your PHP version is 7.3 or above.
As you are unable to access your WordPress admin area so please try to access your file manager in cPanel or access public_html directory via FTP. Plugins and Themes directories are inside wp-content/ directories.
Go to wp-content/ Directory them rename plugins/ directory to pluginsbackup/ now try to load your site. If this works that means you have problem with a plugin.
If problem is with plugin change back the directory name pluginsbackup/ to plugins/ Enter in plugin directory. Now rename each plugins directory by placing backup at end of that directory name.
Once you have changed all plugins directories names, start getting original directory names one by one and make sure you check the website if that’s still working after each plugin start working. As soon as you see which plugin is making problem just delete that plugin.
Source There has been a critical error on your website.
If you can’t figure out what’s wrong please turn on the wp debug mode so you can get details of error. Edit your wp-config.php from WordPress root using FTP. And
Find in wp-config.php
define('WP_DEBUG', false);
And replace with
define('WP_DEBUG', true);
Once you turn on the wp debug mode you are not able to see the details of issue, weather is related to some missing file or some plugin is producing the FATAL error you can track and apply appropriate fix. Thanks. Once problem is fixed don’t forget to turn off the debug mode in wp-config.php