Please Help me hardware question??????

thanks.

I have a question when I do restart my computer the monitor goes black on some occasions and when I turn off the monitor and turn on the monitor I can see the computer Normally.

Sometimes I'm playing or watching anything on the computer and suddenly thescreen goes black but the monitor stays on and keep watching the computer I have to turn off the monitor.

I wonder what you think could be the problem

monitor or videocard?

 

thank you