I had this problem on my Lubuntu system for ages too. I did 2 things to fix it:
- In Light Locker (
Menu > Preferences > Light Locker Settings) make sure that 'blankscreen' and 'switch off display after' are set to 'never'. Then makesure the 'Enable light-locker' switch is OFF. - In XFCE Power Manager (
Menu > Preferences > Power Manager) select the 'On AC' menu, then click the 'monitor' tab. Make sure 'Put display to sleep when computer is inactive for' and 'Switch off display when computer is inactive for' are both set to 'never'. Now do the same for the 'On Battery' menu.
Make sure you hit 'Apply' before closing each window, then reboot your computer and the problem should be fixed. It was for me anyway.