Lower two thirds of screen not showing

Hi guys,

Last night while working on my imac late 2012 21 inch, all of a sudden it went into a flicker and lower two thirds of the screen started alternating between green blue and black color. Thot it was a glitch so i restarted the mac couple of times. But to no avail. And it has not gone completely black in lower two thirds with the remaining one third showing fine lines horizontally and flickering throughout. Can any one help me trouble shoot the issue. How big a hole is it gonna burn in my pocket??

here is a photo grab of the same.

ps: the web page somehow inverted the image.