iMac Turns off immediately after pressing start button

My Imac early 2009 24” turn off immediately after pressing start button i hear the chime but the Imac doesn’t start , i should unplug power cable and restart it with using PRAM command …what is the problem any body can help ?

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1 Comment:

Do you see the display turn on at all?

Can you tell us anything more here like what happened just before you had this problem. Did you smell something burning or hear a pop?