Computer Boots but Screen stays black

Hi ,

I have started to have this strange problem a few days ago . When i start the computer it boots normally but the monitor stays in sleep mode . I can even hear the welcoming windows desktop sound on my speakers . If i try turning pc off through the windows task bar shortcut it even works normally .

switching the monitor off and on . there is a no signal message displayed on the screen then it goes back .

I was wondering if this is a problem with the monitor or the video card .

Thanks .