Disable sleep upon lid close.

https://askubuntu.com/a/372616

Open the /etc/systemd/logind.conf file in a text editor as root, for example,

sudo -H gedit /etc/systemd/logind.conf

If HandleLidSwitch is not set to ignore then change it:

HandleLidSwitch=ignore

Make sure it's not commented out (it is commented out if it is preceded by the symbol #) or add it if it is missing.

Restart the systemd daemon (be aware that this will log you off) with this command:

sudo systemctl restart systemd-logind

or, from 15.04 onwards:

sudo service systemd-logind restart