I have had the same issue. Just turned phone off and waited a few seconds. I then turned it back on by holding the on/off button along with the home button until the apple logo appeared then held the volume up button until the phone restarted correctly (i.e. where passcode required). I have then updated the ios (as I suspect the latest update caused the issue as another one was waiting to be downloaded) and removed the last app I installed to be on the safe side. All working ok now :)