Two black lines in my HP monitor that keeps bugging me

I have HP 2311GT 3D Monitor 23". This is situated in my work room in garage. I would like to know what is my computers problem. It is showing 2 horizontal lines on top most part. I connected this with my home theater system and mixers. I haven’t have any problems for the last few weeks I’ve been using it. But now this is my problem. Every time I turn off my desktop and turned it on, for few minutes I will not see it. After certain time it will show again the black lines. Please help me with this. Thank you!
