On Ubuntu 14.04, use this command to restart the server:
sudo service digits restart
On Ubuntu 16.04, use this command to restart the server:
sudo systemctl restart digits
If you are still encountering problems after restarting, check the logs for errors.
cat /var/log/digits/digits.log
For additional messsages on Ubuntu 14.04:
cat /var/log/upstart/digits.log
For additional messages on Ubuntu 16.04:
journalctl -u digits.service