Solution:
This answer applies if you have Google Analytics for WordPress by MonstorInsights installed.
The problem is caused by one of the latest releases. Their release did not include the file lite/includes/admin/wp-site-health.php.
The solution is to connect to your instance via SSH. Then go to the WordPress Plugin directory /wp-content/plugins and rename the directory google-analytics-for-wordpress. This disables the plugin. You should be able to login to WordPress. Go to plugins and update Google Analytics for WordPress by MonstorInsights.
Update:
I forgot to mention to check the Apache logs. Check the logfile apache2/logs/error_log. Use tail and look at the last couple of errors. This will help you figure out what is going wrong.