Can't adjust screen

the screen on my tv has started to shrink. there is constantly a 2" (approx) black space at the bottom of my screen which causes the viewable screen to warp (scrunch up) and make everyone look like they have short little legs and a normal body.

i have gone through the on screen menu hundreds of times but don't find the vertical or horizontal adjustment. i also looked on the back of the tv but there are no manual controls on the computer.

how can i fix this (the screen is otherwise still in very good shape - good color and contrast).