Try rebooting your phone first by simply turning it off and then back on. If that doesn't fix it, a hard reset could help. To do that, hold down the power button and the home button simultaneously until you see the Apple logo on a black screen. Then allow it to restart on its own. If that still doesn't work, you may need to restore your phone via iTunes. To do so, Google how to restore via iTunes and you will find some very clear instructions for how to do it. If in the end even THAT doesn't work, then you should take the phone to the Genius Bar at the Apple Store. Hope this helps!