Did you purchase a 27″ iMac between May 2011 and October 2012? If so then Apple may owe you a free repair due to faulty AMD Radeon HD 6970M GPUs.
27″ iMacs that were sold between May 2011 and October 2012 – more specifically machines with 3.1GHz Core-i5 and 3.4GHz Core-i7 processors may have faulty AMD Radeon HD 6970M GPUs causing white or blue vertical lines on the display or a solid black display. Free repairs are available for the issue from Apple and additionally, any consumer who has already paid for the GPU fix prior to the free repair program will receive a refund from Apple.