By default, Ubuntu puts your display to sleep after it has been inactive for a period of time. While this is a good idea when you are running on battery power, it can be annoying when you are on AC power.
1. Go to the Main Menu, click System, mouseover Preferences, and select Power Management.
2. Go to the On AC Power tab, go down to the slider control in the Display section.
3. Slide the control to the far right until the word Never appears below it.
4. Click the Close button.