No display,but processor and SMPS fan works

Hello,

I was updating my anti virus suddenly the monitor went off...i thought it was a stand by but even after moved the mouse press keys on keyboard it doesn't work. So I tried to switch off the system by pressing the power on switch for a few seconds, but it doesn't go off, so I switched off the mains and then when I switched it on again to continue my work it does'nt switch on. When I Press the power on button the power light lights up but as soon as I leave the power button, the lights go off but I see the processor cooling fan and smps fan still runs.. But no display and the CD light is lighted up but when I press the eject button it doesn't come out....after half an hour when switched it on the pc works perfectly for a few seconds, the display come for a few seconds but goes back to the previous problem again. Please help me!!!!