I understand. The external display works, therefore the video card in the laptop is OK. The laptop boots up, therefore it works OK (I presume - you haven't mentioned any other problems than with the screen.)
So I have to conclude you have a hardware problem with the built-in screen on your Lenovo laptop. (You could try reinstalling drivers, in Device Manager, for the 'Screen' or 'Monitor' as opposed to for the 'Video card' or 'Video adapter' which is obviously A-OK, but if nothing shows as defective or having a problem or as Disabled in Device Manager, that probably wouldn't help much.) I suspect a 'shut off the screen when it is closed' switch has jammed shut, and thus tells the laptop to leave the screen dark and blank. That can be checked for: go into your Power Options settings, and set the 'shut off screen when closed' to Disabled, and see if that helps... it might override the switch's false indication, and leave the screen on even when the screen is folded down onto the keyboard, but would allow the screen to be used and seen.
If that changes nothing about the problem, then it is likely that your screen is 'broken' in some way... shine a flashlight into it, and see if anything is even faintly visible in it that way, if so, then the backlight is defective. If nothing even with a flashlight, then the cable to the screen may have slipped loose... but these days, it is more likely that something in that cable or in the screen assembly, is quite simply 'broken'.
How old is the Lenovo laptop? if within a year of purchase date, it might still be under warranty.
And of course, if the laptop has ever been dropped or gotten wet or even very damp, that might have damaged the screen function. Let me know, and I will advise you further.