1. Press and hold both the Power and Home button until you see the Apple logo. Afterwards, release the buttons and see if it'll boot to the home screen. If not, it may go into recovery mode and display an iTunes logo and an icon of a USB cord,
2. If iTunes detects the device in recovery mode, restore it via iTunes. Make sure the computer is connected to the internet.
3. If the restore fail, try again. If it fails a second time, download the .ipsw file for your specific device via the following link:
iClarified - iPhone - Where To Download iPhone Firmware Files From
4. Put the device in DFU mode, and then direct iTunes to install the .ipsw file you downloaded.