Issue description
If you are in 3D and you see a large flat blue grid, this is called the Editing Plane. It is an on-screen aid for working in 3D.
If you want to turn it off but don't know how, read this article for the instructions.
Solution
You can change the color/opacity of the plane if you are in a 3D view and go to View > Grid & Editing Plane Options > Grids & Background....
To toggle it off completely, go to View > Editing Plane Display.