Ok, so I am taking a random guess at this.

Your screen DOESN'T go on when you turn on your laptop?

If that's the problem, try moving your screen a bit and see if that helps. If it does there's some wiring loose in which case you'd need to open it up ( or send it to a shop ).

If that doesn't work, your screen could be broken but it could be your GPU aswell. Depending on what laptop you have, you can try to use the video out or HDMI out and plug it into another screen ( doesn't always work ). If it works then, you can be sure it's your screen.

EDIT: It is unlikely that it is your GPU, as your screen would go on but you'd be getting a weird image ( 8-bit )

Your screen mabye be faulty,

Wiggle the scrteena bout or if not mabye read the manual.

i have tried with another screen but i cant choose it as my default screen i dont know wether its the magnet in the screen when i is closed and opened

Make sure its not in presentation mode mate!

TTG Zac 8)

It could also be the sensor inside the screen, as you said.

To try and fix that, you'll need to know if it's a visual sensor or magnetized sensor.

Visual: Tap it gently ( as their might be dust inside the lens ) and clean it with a soft cloth.
Magnetized: Not really a safe way to do that as far as I'm concerned, as using a different magnet to get it loose may de-magnetize it, in which case it won't ever work again