Problem with monitor/Horizontal line

[zipcode1]

I have a 2009 Mac OS X 10.6.8. Out of nothing, a thin horizontal line appeared on my screen. Does anybody know what could be the problem?
Thank you in advance!

 

[smirk]

You'll have to give more information if you want any help. Is this an iMac or a 3rd party display? CRT or LCD? Does the line always appear regardless of what's on the screen? Does it always stay in the same place on the screen? Does the line extend all the way from the left edge to the right edge? What color is the line?

 

[zipcode1]

You'll have to give more information if you want any help. Is this an iMac or a 3rd party display? CRT or LCD? Does the line always appear regardless of what's on the screen? Does it always stay in the same place on the screen? Does the line extend all the way from the left edge to the right edge? What color is the line?

It is a iMac, LCD. The line appears always regardless of what's on the screen, it stays always in the same place and it extends from the left to the right edges. The line is very thin and gray and it delineates the lower third of the screen.

I hooked up another monitor to my computer. The line does not not show on the other screen.

Thank you!