Cpu works fine,but monitor is black..........

Hello,
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hi i hv the problem with my pc whn i turn it on........the whole cpu is working........cd rom is also working.........all fans r running......all lights r there..........but there is nothing on the monitor.........it remains black.........after beating it smetimes.......it starts......and again aftr switching it off.......and turning on aftr 2 3 hrs same prob occurs..........one time i gt irql not less nor equal message...........on blue screen.......but dont know wht to do...........i hv tried my cpu on other monitor too but nthgn worked....changed my ram tooo..........but problem is still there.................anyone help plz...............plllllllzzzzzzzzzzzzzzzzzzzzzzzzzzz1!!!!!!!!!!!!!!!!!!!!!!!!!