Display drivers installed properly? Thought: Could also be an old bios not being updated, causing display issues.

It always worked since I had laptop 3 years ago until recently. Just out of blue not working.

Tried D + Power to trigger display diagnostic. Screen responds with colour test. So I think the screen is ok. 

So the screen works when you are in the BIOS or?

Screen works. Can't get to bio. Tried F12, F2, Fn+power already. 

Thanks for following up. The front LED light is white. Keyboard is backlit. Cap lock light is on but would turn off after 10 sec (and won't turn on even if press). Unless there is a way to trigger e-bio recovery or a recovery via USB, I'm really stuck.

So does the laptop still have any specific diagnostic indicators active?

Yes. I've tried the flea procedures of removing battery and CMO coin battery. Upon restarting, it gives 3 orange and 1 white, which eventually goes away and back the just solid white (but still stuck without Dell splash screen).

I've tried different CMOS battery to no avail. 

Sounds like you'll probably have to either ship it to Dell for them to troubleshoot or have Dell send a technician (who will probably replace the motherboard).

Thanks. I fear as much. Seems like a common problem with XPS