mac wont turn on after reflow of the video card?

hey guys i need help my imac late 2009 27 inch was dying. Since i did not have money to get it fixed i tried a method to reflow the video card after when reflow was done I reassembled the computer i head the chime but the screen remained black. I thought i did not reconnect the display cable properly but after rechecking all the cables, everything was connected properly. So my question is how can i fix the card? And if i cant fix it how can i buy new one? I hope someone can help me with this. Thanks