MySQL or Mariadb (probably mariadb, but that depends on when you installed Virtualmin, on Ubuntu) is not running.
Try to start it, and see what errors you get.
systemctl restart mysql
Then check status:
systemctl status mysql
If it is running, your apps should be working again. If it is not running, you need to figure out why. Look at (and actually read) the messages on the status, and then also check the journal for the mysql unit.
journalctl --since=yesterday -u mysql