Black screen and system frozen switching console -> X

[Yahawas]

Hi all, I'm stuck with this problem.
I can switch to console using CTRL+ALT+F1.
When I switch back to the X server pressing ALT+F3, the screen is black and the system gets totally frozen.

No way to restore it, either pressing CTRL+ALT+DEL or BACKSPACE.
Does anyone else have this problem?

Thanks