Please ask your host to check the database. Normally that's a crash. https://dev.mysql.com/doc/refman/8.0/en/gone-away.html
Hi Donna
Upgrade completed, FastComet host found:
Hello again, Thank you for your patience up to now. After further investigation it occur that the database adapter in the configuration file is no set correct. In order to utilize PHP 7+ versions the MySQL adapter should be pdo_mysql. As soon as I have performed that change I am able to see your website loading properly.
Thanks Gary
I was looking at this again and it appears that they are not updating the servers to the requirements. You shouldn't need or be using the pdo_mysql as you should be using MySQLi as our requirements show:
https://www.socialengine.com/support/article/81788966/se-php-socialengine-requirements
They might want to check all of the new requirements.