How do I prevent Ubuntu from putting my monitor to sleep?

Things have changed since this post was submitted. Go to System Settings in the top right corner of your screen, select Brightness and Lock and set "turn screen off when inactive" to never.

hope this helps all future users.

Check System>Preferences>Screensaver and see if "Activate screensaver when computer is idle" is checked.

Also, if you hit Alt + F2 and enter gconf-editor then navigate to /apps/gnome-power-manager/lock you can see if blank screen is checked.

You can use gsettings for enabling and disabling monitor from going to sleep:

• Disable turning off screen

  gsettings set org.gnome.desktop.session idle-delay 0

• Enable turning off screen with 5 min delay

  gsettings set org.gnome.desktop.session idle-delay 300

source : https://askubuntu.com/a/788456/88543