XPS 15 9550 screen went black but external monitor works

I have a Dell XPS 9550 running Windows 10. Yesterday all of a sudden the LCD monitor went totally black, and now it won't turn on at all (not even the backlight), but an external HDMI monitor works fine. I've tried all the usual: powering it off and on, running diagnostics, booting into safe mode, booting into the BIOS, etc, and nothing I've tried will make the LCD monitor working again.

Additional notes:

* I can run the display test on startup by pressing "D" and it runs fine -- I see red, green, blue etc -- and the monitor itself seems to be working fine. So it's not an issue with the LCD monitor hardware.

* Even when I boot into the BIOS (either via F12 on startup or via rebooting into the UEFI from Windows) it doesn't turn on the backlight or show on the LCD screen. But I can use the BIOS fine via the external HDMI monitor. So it's not a Windows or driver issue, as the BIOS (well before Windows boots) has the same issue.

* I've tried disabling the Intel and the Nvidia graphics cards in turn, but neither operation helped (and see above point where it doesn't seem like a driver issue).

* The touch aspect of the LCD screen still works.

* I've tried updating to the previous version of the BIOS (1.8.0) and it updates to that version but doesn't fix the problem or change anything.

* My laptop is not under warranty so I want to try to fix it myself.

Any ideas of what to try next? Maybe the red/green/blue diagnostic test uses lower-level hardware to drive it than normal, which is why the "D" test works but nothing else?