|"Try pressing Fn+F7."|
If that doesn't work,
- the Fx key you use , where x is a number, often has an icon on it that looks like a monitor screen
- or - in any case, see the Owner's or User's manual for the specfic model
Have you, or is it possible someone else who had access to this computer...
- dropped it, or dropped anything on it, or otherwise exposed it to a physical jolt ?
If yes, there may be something else damaged inside the computer.