The system hangs up when I press ctrl+alt+f1

[myusernet]

I want to kill X, so I press the combination but the system hangs up, half of 
the screen was the desktop and the other one is totally black. I tried alt+f2, 
the light of num lock on the keyboard was off, and after pressing alt+f1, the 
light was on again, so, I think the keyboard was still working. Alt+F7 cannot 
bring me back to the desktop. What should I do?