Laptop screen doesn't come on!

Well heres the story..

The d/c jack on my laptop broke, i bought a new one off ebay and now it will charge ect.

The night before i plugged the new d/c jack in, i took the screen out of my laptop and tried it in a different laptop (because the other ones screen was broke and i was testing nothing else broke)

But now when i have put my laptop back together, the laptop powers on and loads windows as normally (presumably - hdd light starts flashing ect) but nothing is on the screen.

What could be the problem?
 
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