Restarting your phone is the first thing you should do in this situation. As easy as it may appear, this may occasionally solve a problem. To restart your phone, simultaneously press the Sleep/Wake and Home keys until the Apple logo displays.
If the problem persists, you'll need to back up your phone's data, then restore it and set it up as a new device. Do not install any apps on your phone until this is completed. You should first check to see whether the issue persists.
If the previous methods do not resolve the issue, it is possible that the problem is due to a malfunctioning display. I propose that you take it to a service centre to get it looked at.
