I just built my first computer, and everything is running great. I have ran into this problem 3 times where if I choose to restart, before restarting screen will go to a black screen with a C9 text in the bottom right. I have to hold the power button to turn off.
Also this happened as well over this past night after downloading some games, and in the morning my screen was black with the same C9 text in the bottom right.
When I hold the power button and restart, everything boots up great.