Why is this laptop display black?

A customer brought in the their Lenovo Thinkpad 450s because their screen was failing.
They already had purchased a new screen and gave me the e-mail confirmation from the order that showed that it was the correct screen that they needed.

However, the new display is still black.
When I hook up an external monitor, nothing but a mouse pointer shows.

But I can connect to the external monitor if I power on the laptop and then immediately close the lid.
The external display then shows the desktop.

But then I lift the laptop lid, the laptop screen is still black. Even when I change the display settings to extend the desktop.

Any thoughts?

Added (1). There was one brief occurrence with the new screen, where I saw a very faint lenovo logo appear when the splash screen loaded. But then it went completely black again. And has not done even the faint image since, it was just a fluke occurrence.
And when I shine a flashlight, I still see no display.

Added (2). While the external monitor is displaying the desktop and I then reopen the laptop lid, the laptop screen is recognized as being there. ( Display settings, shows monitor 1 and 2 ).
But the laptop screen is still black.

Caleb

Ok close the lid so enables you to use the monitor… Now control panel / hardware sound / power options / choose what closing the lid does… That will resolve that particular problem… As for the black screen on the laptop check the inverter / shine a light to see if just dim… Check the video cable and connector, try reseating and rebooting… If still black, disconnect external monitor, power down and remove the battery and unclip cmos, hold the power button in for a minute then try connecting cmos and booting without the battery and use the psu

Paul

The FN and screen button! Push FN and the screen button uptop were the FN buttons are the screen switch button on a Lenovo is marked.

If you can start the computer and see the bios boot screen there's nothing wrong with the screen if you can't then it's the inverter chip that drives the screen

ari