Elitebook 820 powers up but blank black screen

[brianst0rm]

I have an HP Elitebook 820 G3, with Windows 11 installed on its 256GB SSD.

The computer appears to power up.  With the AC adapter plugged in, the 'battery light' on the front edge of the notebook shows amber.

When the power button is pressed, the keyboard backlight illuminates, and the 'power' light and 'wireless' indicator lights on the front edge also both light up.  But the screen remains dark:  not even the HP logo.  After approx 30 seconds, the keyboard illumination goes out, but the three indicator lights on the front edge remain lit.  Pressing the power button causes those three lights to go out (ie the computer shuts down).

I have tried the simple re-set and check, unplugging the AC adapter, holding the power button down for 20 secs, then reconnecting the adapter and powering up.  Same behaviour as before:  keyboard and indicator lights illuminate,  but nothing on screen.

So I then did the same check, but this time on re-starting pressed the power button and repeatedly the 'esc' button at the same time.  Again, keyboard and indicator lights come on, but nothing visible (not even the BIOS display) on the screen .... so no way to choose F2 or any other option.

UPDATE:  I have done one further test.  I connected an external monitor via the VGA port.  After some switching on and off of both laptop and monitor, a full display appeared on the monitor screen (only).  This enabled me to check the system.  The computer itself seems to be working normally.  I ran the HP Diagnostics app, made sure all drivers were updated, and looked at the Display properties.  It listed both the notebook's "internal" display screen, and the external monitor, and appeared to say that both were working.  But it proved impossible to switch between them or extend the display to both.  Only the external monitor operated.  I tried to adjust the brightness of the external monitor:  moving the slider seemed to make no difference.  I ran all the HP Diagnostic system tests.  All passed except the Video I/O test:  but no further advice or checks were suggested, so I still don't have a diagnosis.

I subsequently tried to reboot the computer again, without the external monitor.  Power plus esc plus F2 appears to produce the faintest outline on the notebook screen of the Startup Menu - but so dark that it's almost just a shadow.  Fn plus f5 and f6 has no effect on screen-brightness.

Do these symptoms mean the screen has failed?  Or the graphics driver?  What other checks, tests or experiments can I do? 

[brianst0rm]

Ten days since I posed a help request.... and not a single reply.  Leaves me wondering how active, and how useful, the HP community forums actually are.

I post again simply in order to say that - without any advice from this forum - I have identified and resolved the problem.  Connection to an external monitor confirmed that the motherboard, graphics drivers etc were all good.  But the [original] OEM screen was faulty.  I replaced the screen (which had a crack hidden behind the bezel along the lower edge) and the computer, with new screen works perfectly again.