I know this will sound silly but it works. There is a grid under the touchscreen that is held in place by glue. Over time the glue starts to separate from the screen and the screen becomes non-responsive.
Here is the simple fix. You need to re-heat the glue so it sticks to the grid. Here is how you do it. First remove the battery and sim card. Next you'll need a toaster oven. Now looking inside the toaster oven, look for the heating elements (bars) inside at the top of the oven. Be sure to place your phone FACE DOWN over a napkin on TOP OF THE OVEN (NOT INSIDE) so that the phone is centered evenly over the heating element. Turn on the oven at 450 degrees for 10 minutes. IMPORTANT!! Let the phone cool off FACE DOWN FOR TEN MINUTES before you re-install the battery and sim card. The reason being that the now heated soft glue will need to cool down and reattach itself to the grid and touchscreen. If you don't center the phone correctly, you'll notice that only one side of the touchscreen will work. If this happens, no worries, you can do this again till you get it right.