Most likely the either the LCD display itself, or the printed circuit board has developed a crack, and some of the signals that drive the display have been interrupted.
To inspect the printed circuit board, follow [guide|34616|this guide] (basically taking the rear cover off is all it takes).
The LCD is not repairable. The printed circuit board may be repaired by someone who is reasonably skilled with a solder iron, the operation entails bridging the interrupted circuit tracks with pieces of thin copper wire.