Here's a strange one! I have an elephone P6000 running lollipop five. Yesterday, I got a text which, when I tried to reply to it, found that the reply text box wasn't accessible? I could see the box,, but no matter how I tried, I couldn't activate it to pop up and present the reply keyboard. It's not a screen issue, as that part of the screen is usable on many other apps. I have since found I can't use WhatsApp for the same reason. Also, if I go to settings, the very last item is 'about phone', this also doesn't respond to taps ( yet if you slide the menu down one item, you CAN access the next item by tapping the bottom of the screen????) Any ideas please?
Have you tried just rebooting the phone? If that doesn't clear the problem, try a factory reset. And if that doesn't fix it, then it's almost certainly a hardware problem with the touchscreen itself. Which requires repair or replacement.