ps aux |grep mysqld
from terminal. Or use whatever tool you might have to check running processes.
If not running, start the db server daemon.
Assumes DB server is not remote (if it says 'localhost' for DB server in config.php file then DB server on the same server as code and above command will show if it's running)
Check that the DB server is among the daemons to start on reboot.
Otherwise, you will have to provide some more info:
how moodle installed would be a good start.