You probably switched to a 3D view like one of the Elevation views. You should be able to go up to View, and then click Plan. There could also be a toolbar button to switch to Plan view depending on your setup.
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)
I see this was from several months ago, but I only saw the original post today. If you didn't find out what you did to cause the problem, it might be that you accidentally touched the F12 key. If so, those vertical lines were the Z value of the entities that you have placed in plan. As for how to get out of that, simply go to the menu bar at the top of the page and pick View. A fly-out will show Plan as the first option. You can pick that and all should be well. If your Plan selection shows a Ctrl+ key option you can use that if it ever happens again. Mine is Ctrl+P. I don't know if back in the dawn of time, I added that to my options or if that came in DataCAD and I just started using it whenever I messed up and touched the F12 key on a strange keyboard.