No display and 4th LED Not on

Hi,

When i plug in my imac 1st led comes then after pressing power button i hear half chime sound (sometimes no sound at all). I can hear the drive spinning when i press power button. 2nd and 3rd led also stays on after starting the system. But 4th LED never turns on.

History,

I was using my imac as normal and i felt it was working slow, alot of hard drive noise. I went out n came back and system was restarting. And it stayed on boot screen for 5 mins. I restarted it couple of times and it never went pass the boot screen. Eventually for 5th time the screen was completely gone.