You're encountering the clipping plane, which is just the "front" of the 3D modeling space.
To readjust your view, select the View Move tool and click and drag one of the corner handles down. This will back up your view so that all of your model is in the view.
You could also use Fit Views to All, Fit Views to Selection, Fit Views to Active Grid or Reset View from the Windows menu to reset your view.
Reset Views does not change the position of the view, however it will clean up any corruption to the modeling view that may have occurred while zooming or adjust the view position and/or rotation.