I had a problem with my NoMachine Programm so I decided to use the possible solutions from their website: https://knowledgebase.nomachine.com/AR03P00973
I tried the commands in the graphical desktop environment:
sudo systemctl stop lightdm and sudo systemctl stop display-manager and both put me in a blackscreen where I couldn't do anything except rebooting my PC. After I rebooted my PC there was no blackscreen anymore. Then I tried the command: sudo systemctl stop gdm and after this command I'm still in the blackscreen even after rebooting. And I also cant see my cursor. I can't even get to the Bios because the screen is just black. Maybe switching between the TTY screen is working but I cant tell since I cant see what is happening. I dont know what to do. I now know that I shouldn't have used these commands in the normal graphical desktop environment.

I'm using the Version 20.04.3

I'm afraid none of these solutions helped. I turned my PC on tried all the TTY screens but all of them gave me a black screen. I even tried to log in, open the console, and type in the command for restarting the display-manager but no matter what I did there was still no video signal. I'm using the version: 20.04.3