Black screen on laptop screen after boot

See if a screen diagnostic will run - hold the D key through powerup.

@ejn63​ i have tried it the it shows the red green and blue light 

That means the display is OK and receiving power, and that it's likely a mainboard fault.

Anything more detailed requires the system model information.  If you have an external monitor, you should also check for an image on that.

@ejn63​ okay thank you but how can I fix it 

@user_046586​ it is a dell latitude 5490 8th Gen

You can try system reset (unplug, disconnect the battery from the mainboard and hold the power button for 30 sec.  Remove and reinstall the memory module(s) and hard drive or SSD.

Then reassemble and see if the system will boot.

@ejn63​ okay thank you so much