I would do the following before considering a screen replacement.
* Restart the phone
* Inspect the cable, the cable should be accessible without unscrewing the back. Remove the back case and on the upper left corner you will see a plastic piece with a little slit that you can pry open. Remove the battery, pop that chamber open, and disconnect the cable and look for signs of damage or corrosion. If you clean the cable or no signs of damage are evident then you may need to replace the LCD/Digitizer.
If the issue happened after the unit was dropped then more or less it may have suffered damage and the digitizer is affected.
Good Luck