i said "i use ctrl+alt+f6 then ctrl+alt+f7" . edit : i use ctrl+alt+f7
then ctrl+alt+f8 .
i said that i searched for "wake black" , but now i have found out
that mere ("mere" meanining "only") locking screen with ctrl+alt+L
also causes same black/turn off screen. (it stays black for some
little time, then command prompt appear for very little time, then
screen turns off).