Just bought a second-hand iMac G5 20" but it won't start up. when I push the start-up button it shows the power light and after some seconds the fan starts but no sound and the screen remains black. When hold the power button to off and on it again the fan starts after some seconds but the power light will not shine and the screen remains black. when i unplug and plug it again the the power light come on and the fan after some seconds and the screen remains black. I put a HD, but it remains the same. PLEASE HELP.