Replaced screen and still a black screen is showing.

assuming you still get the display over to the external monitor, i have a couple possibilities

1) cable disconnecting while mounting the screen: Open up the device, and make sure the cable stayed connected while installing the screen. They use a pretty tough tape to keep it installed, and sometimes it doesnt hold after tampering.

2) screen they sent you was faulty: Check the warranty policy of your screen, and see if they can send you a second one.

3) device was operated on with battery in: If the device was operated on with the battery in, the power source could have fried out the LCD or the GPU. or sparks could have melted the mounting plastic on the connectors

4) connector on either the cable or screen got damaged: make sure there is absolutely zero physical damage to the connectors on both parts. (broken corners, bent pins, etc) a microscope comes in handy for this

Hi @petrasanta61182

Please confirm the model number of your laptop?

If there is a display when using an external monitor connected to the laptop, try shining a flashlight at an angle close to the laptop screen (with the display switched back to the laptop screen, away from the external monitor) and check if you can see images on the screen.

They will be very faint so do this in a darkened room as it may help to see them if they are there.

If you can see images then there's a backlight problem in the laptop.

Knowing the model number will help to find out whether it is a motherboard problem, a LVDS cable problem or a problem in the screen itself.

When working inside a laptop, always disconnect the battery from the motherboard as soon as possible before doing anything else. There is always power on the motherboard somewhere. The Power button is not a power isolating button. Think of the laptop as being in an extremely low power state when it is off and not that it is totally disconnected from any power at all.