The display back light doesn't work all the time when logging in?

When the laptop has been closed and I open it the screen appears to be just black but if you look closely enough you can see the log in box. If the cursor is in the right place then I can log in but other wise I have to keep firmly opening and closing the laptop until the screen lights up. If that doesn't work after a few attempts then I have to hold down the power key.

What could this be as it is very intermittent?

Nothing has been spilt on the laptop and nothing else appears to have been broken.