Black Screen

I run my computer in Win 7. I have work for 3 hours until I am very tired and take a rest for about 90 minutes. When i am back in front of my computer, the screen is gong black and the pilot lamp/power lamp on monitor is blinking like no signal on monitor. I thing it,s normal. So I move my mouse on the padmouse, ussualy after a minutes monitor screen wake up again. But not hapen for this time, even when I hit enter, hit ctrl alt del, nothing happen. I try to press reset button, it just same (no reaction). I look to pilot lamp on cpu and I see my hardisk is working.
Why my computer look like stiil working but no display in my monitor ??
At last I press "of button" on my cpu so my comp restart and comp run normaly again. What wrong with my computer ??

Please follow the https://www.sevenforums.com/crashes-d...tructions.html to provide us with more information about your system including your system logs. The system logs can help with determining reasons why the system will not start up properly. Those instructions are not just for blue screen crashes.

Thank you. :)