When the local X server cannot be found, NoMachine is able to use its own display service (that is an embedded X server) to let users connect seamlessly a physical desktop running in background on the remote machine.
It can happen, depending on the video card, that the X server is running but it is not rendering because the video card is turned off. In this case NoMachine detects the X server and doesn't use its own display service. As a result, the NoMachine session shows only a black screen.
1) Try to stop the X server manually.
Use the proper command to stop the X server accoding to your display manager, for example:
sudo service lightdm stop
sudo service gdm stop
sudo service kdm stop
and restart the NoMachine server:
sudo /etc/NX/nxserver –restart
At this point NoMachine will create its own virtual framebuffer and you should be able to connect and see the desktop.
2) As an alternative to 1), reboot the machine at runlevel 3:
sudo init 3
3) For nVidia GPUs, another possibility is to configure X.org to run headless:
4) A further option is to adopt fake display dongles can be inserted in the video card ports to simulate the presence of a screen.