Ubuntu 16.04
Today I can't ssh to a Ubuntu server (named alpha) on QingCloud,
and can't connect to it via ping command.
The VPN is good because I can access web service on another host
in the same LAN on QingCloud.
The alpha host itself is OK because I can login to it via the web
console of QingCloud.
After running command ping baidu.com, I got network unreachable.
So it's clear that its network service is down.
To restart the network service, run:
sudo systemctl restart NetworkManager.service.
It works. Now I can logon this host via ssh.
Ubuntu 14.04
Sometimes the Dell laptop can't connect to WiFi,
run sudo service network-manager restart and it then connect successfully.