Why does my internal laptop screen (Lenovo IdeaPad Slim 5i) stay black and only works with an external monitor?

I’m experiencing an issue with my Lenovo IdeaPad Slim 5i laptop.

The laptop boots and works normally only when connected to an external monitor, but the built-in screen remains completely black.

The situation is as follows: the laptop works perfectly with an external monitor, internal laptop screen stays black at all times, i tried display shortcut keys (Fn + display key) with no effect and Windows display settings did not restore the internal screen.

I tried to restart, switching display modes (Duplicate / Extend), uninstalling and reinstalling graphics drivers and checking brightness and power settings.

Is there any additional diagnostic step I should try before opening the laptop or taking it to a repair shop?