I am new to Ubuntu. Just installed Ubuntu server 16.04 onto Dell R730 server yesterday. The first time reboot was okay. But the server refused to boot today. The error message was: Failed to start LXD.

Can anyone please help?
This problem is after upgrade. LXD service needs to be restarted using the root account.
sudo service lxd restart
You have to reboot afterwards, e.g. using this command:
sudo reboot