Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
This usually happens if the firmware/system software is glitched. First we can reset your iPhone and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot. Then try to unlock it.
Please let me know if that didn't work, so we can continue troubleshooting.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.