Try grabbing it with your mouses cursor on the top edge and pull it down towards the bottom of the screen. You know you have it when you get a double-headed arrow on the edge of the blue line.
I have a blue line 3.5 inches wide on my desktop that I can not get rid of ! It has I think something to do with the task bar, because it has folder tasks,other places,and details bar in the line . The line runs up and down the desktop

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic