First and foremost, don’t freak out! This is a common problem, especially on the most recent iOS version. Follow these steps below to fix your problem:
Step 1) Restart your iPhone by holding down the home and the sleep/wake button at the same time until the Apple logo appears.
Step 2) If that does not work, you will have to put your iPhone in DFU mode by following the guide here.
Step 3) After putting your iPhone in DFU mode, you will actually have to restore it back to factory settings. The instructions for this process can be found here.
Hope this helps you guys!