I have the same problem.
I installed Ubuntu 20.04 with iso image and everything worked fine.
All of a sudden the server is on and i have constanly black image in the monitor. I checked vga cable, keyboard and mouse in another devices and work properly, on the server are dead.
I tried with the above solutions…Alt + Ctrl + F1,2,3,4,5,6,7,8,9 and nothing happened.
I have to mention that i didnt change anything.