Often times the key pad could break or become worn. This repair guide will show you how to replace the the keypad of the LG VX5400.
Turn off the cell phone and turn it over on its back side
Hold the phone case in your palm and slide the back cover towards yourself using your thumb
Slide a tool or finger into the notch at the top of the battery to carefully remove it.
Remove the rubber stopper from the back top of the phone with the pointed end of a spudger.
Use the Phillip screw driver to remove the hidden screw, which is marked by the red square
Gently split open the front and back of the bottom casing of the phone with fingers.
Carefully pull the key pad from the phone through the opening.
See here for parts that have been removed.