Hold down both the home button and the power button until the screen turn off and then back on with the white apple logo on the screen, once you see the apple logo, release both buttons.
Basically you keep both keys down for about 10s
Connect your phone to iTunes and sync it. Once the syncing finish, disconnect and reboot your phone. If it doesn't work after that, then restore the iOS via iTunes. Remember to backup your files and data before restore the iOS.