why do i have a grey thick horizontal line on the bottom of my screen imac

hello

I been having a problem with my iMac 21.5 inch mid 2011 since this monday January 7,2014. Theres a grey and some times black thick horizontal line at the bottom of the screen, I cant see my programs at the bottom of the screen. First its black then turns into grey and it starts flickering and then it stops flickering and it wont go unless I turn off my mac or put it to sleep, and then like an hour it comes back again. please could you help me Thank you.