You can switch your screen on or off in a terminal when using the xset command in Ubuntu Linux:

to query information about your screen simply type the command:
# xset -display :0.0 -q

to turn your screen to standby or off type:
# xset -display :0.0 dpms force standby
or
# xset -display :0.0 dpms force off

to turn your screen on again simply type:
# xset -display :0.0 dpms force on

in case the dpms should be disabled on your screen, you can enable it manually with:
# xset -display :0.0 +dpms

Note:

If the screen stays black after switching the screen on again use this command:
# xset -display :0.0 s reset