After yuo've switched it on and it doesn't work, try pressing reset, see if anything happens.
Does the monitor make any changes at all? Like does the green light come on and then go back to standby or anything? Can you test the monitor in another PC, or try a different monitor?
Does it sound like everything's normal? I.e. does it sound like it's booting windows (you can hear the hard drive click for about a minute after you turn it on), or does it just stop and you can only hear the fans?
Have you changed any hardware at all recently, or moved the PC (dropped it?)?