The top row of the keyboard of my iPhone 4S, in the password screen, is not responding to my fingers. How do I fix this?

iPhone 4S

IOS 7.1.2

This problem occurred after a whole days use of the iPhone without any hitches. I first discovered the problem by unlocking the phone and then swiping to the password screen to find that the top row was not responding to my fingers at all. Because of this I could not type in my password to unlock my phone. I then tried to restore my phone but could not because it needed the phone to be unlocked. I then tried holding down the home and sleep button at the same time until it turned off and on again, but this did not fix the problem. I also tried switching on the Voice Over mode on and trying to unlock the phone through that but of course it didn't work. I have also left it overnight but I was still unable to unlock the phone the next day. Any suggestions on how to fix this would be greatly appreciated.