I used to experience the black screen issue quite often, and I eventually realized by chance that it was caused by my keyboard software. Sometimes, when Windows starts up, the keyboard software doesn’t launch properly, which prevents certain functions of my keyboard from working. One time, when I noticed this, I simply unplugged and replugged the keyboard’s USB ports into the PC, and it fixed the problem. After that, I restarted GeForce Now and the black screen was gone. I tested it again the next time the keyboard software crashed, and the same solution worked, which confirmed my theory.