Display on for a split seconds then off again. 3rd led off

Hi guys i got this imac from my sister after it has been in storage for a while.

So the 3rd led is off, and as i understand it is a grapics card problem. But funny enough when it boots the screen comes on for a split second. After it boots if press the power button and put the unit to sleep and press power button again the screen comes on for a second again and i can see the logging screen but then it goes off.

So took it apart to see if there is any loose connection and all seem to be fine. If you guys have any idea what it could be that will be great.

Thanks in advance.