What you would need to do is follow the guide (link below) in order to get it open then find where the digitize cable plugs into the board. Look for corrosion and clean it off with isopropyl alcohol and a toothbrush. Let it dry and turn it back on to see if its working again. If not the only other step is to plug in a new working screen and see if it is a problem with your current screen or if there is board damage causing the issue.
Here is the tear down link to get inside: [guide|19016]
Hope this helps.