The problem looks like your database server is not running and your web server is consequently unable to access it. Unfortunately, it is impossible to tell why your database server is not running without logging into your server.

You need to log into your server via SSH and find out why your database server is not running. If you are using an up to date version of Debian/Ubuntu/Fedora then the following command:

sudo systemctl status mysql.service

Will show you the current status. You may simply need to restart with the following:

sudo systemctl restart mysql.service

or the issue may be something else. In any case, these two commands should give you enough information to resolve the issue.