If you shine a flashlight at the screen can you still see the images on the screen or is it completely black? Trying to determine if it is the LED backlight or the whole display.
Sounds like screen sleep. Check your Energy Saver system preferences to see if you set screen sleep too short. Also check Exposé system preferences to make sure you didn't define a screen hot corner to put the screen to sleep and you are "wondering" into that area unintentionally.