Both a Dell Latitude 5420 (Windows 10 Enterprise, Core i5-1145G7) and 5430 (Windows 11 Pro) shows a very strange symptom. After switching on, the BIOS screen shows up normally on the built-in LCD display. The screen goes blank when Windows starts booting. An external monitor helps, Windows shows up on the external monitor, no matter if it is plugged in during Windows startup or at the end when Windows is already running. If the external display is connected during Windows startup, then the startup process is shown there. The built-in LCD is always blank during and after Windows startup. External (HDMI and USB-3) monitors work fine, even together as dual display. Trying to detect the built in display does not help: Settings / Display / Multiple displays / Detect results in "Didn't detect another display". Both machines have Intel(R) Iris(R) Xe Graphics.
How did I get here?
For the brand new Dell Latitude 5430, the mistake was attaching an external display via an USB3 - HDMI adapter during the first installation (power was delivered via this adapter). The installation process was done on the external monitor. Re-installing windows from the recovery partition did not help, at least the official way, using the Dell restore utility in Windows. After that, I was trying to boot Ubuntu Linux from a pen drive. It crashed and during the next boot some kind of system check was starting (checking drive C:, etc). This recovery process somehow fixed the problem, made the built-in display available. My idea was that some display setting might have been preserved in the EFI partition for the Windows boot and it has been fixed during the crash recovery, but I can't check this theory.
The older Dell Latitude 5420 was working fine with the built-in LCD and two external (USB3 and HDMI) monitors for a year or so. Today morning it was booting up (from hibernation) and the USB3 - HDMI adapter was plugged in. It resulted in the very same symptom: the built-in LCD goes blank after the BIOS screen and there is no way to switch it on. In this machine, I have read-only BIOS access only and can't boot from any other media. System admins may help if we can figure out what to do. Any ideas how to fix this problem?
I have read and tried DELL latitude shows a blank screen unless connected to external monitor and Dell Latitude E6410 blank screen but it did not help.