Hello to all members.
I am trying to use a Rock 4 SE. At first everything was fine, but only for a few days. Now the monitor stays black all the time. I have written the micro SD several times with Balena Etcher and also with Rufus, I have tried changing micro SD and power supply, I have tried different HDMI cables and different monitors, I have tried different images of different operating systems but it is impossible to display anything on the monitor. I can connect to the SBC through the serial port and, with some operating systems, with SSL, and it seems to work fine, but the graphics server does not work. I have tried a second Rock 4 SE but I have the same problem.
Thank you to everyone who will give me a suggestion
Rock 4 SE: monitor always black
Problem solved: Samsung monitors are not compatible with sbc (and this is not the only problem). 
Changed the monitor, problems disappeared!
ich have also some trouble with a old phillips TV-Screen with a resolution of 1024x768. If your using xfce desktop you can create the file: "/etc/X11/xorg.conf.d/10-monitor.conf with following content:
Section “Monitor”
Identifier “HDMI-1”
Option “PreferredMode” “1024x768”
EndSection
Of course you have to specify the right Screen resolution of your Screen.
I have solved my issues, but i only tested it with xfce desktop environment.