How do I re-enable the built-in laptop display?

My cousin has a Presario CQ57.  Her screen is black. Computer running. She thinks her daughter disabled it at keyboard. What keys do you press to turn screen back on?

Oron J 2014-08-16 08:20:31 It's Fn + F4 for your model. Hovsep A 2014-08-16 08:15:36 HP Compaq Presario,Presario CQ57-210 Getting Started Manual: Using The Keyboard; Identifying The Hotkeys Bruce E 2014-08-16 06:16:49 Most laptops use Fn+F4 to toggle among the display outputs. You should also check the appropriate ones for the display brightness as well. I believe these are Fn+F2 and Fn+F3 on the CQ57 which should increase and decrease the brightness, although they are Fn+F4, Fn+F7 & Fn+F8, respectively, on a Presario 900 and Fn+F3, Fn+F7 & Fn+F8 on the Presario 1200. At the lowest brightness setting, the screen can appear to be black.The icon for the display toggle looks like 2 screens separated by a vertical bar on most laptops. The brightness icons on Compaq devices tend to be a circle with rays emanating from it with a small triangle to the right either pointing up or down.