You can enter DFU mode to fix [http://www.transfer-iphone-recovery.com/iphone-stuck-in-recovery-mode.html|iPhone stuck in recovery mode] or restore mode:
# Plug your iPhone into your computer with a USB cable.
# Turn off the iPhone.
# Hold the Power button for 3 seconds.
# Hold the Home and Power buttons for 10 seconds.
# Release the Power button but keep holding the Home button.
# After about 15 seconds you will be alerted by iTunes saying that it has detected a device in Recovery Mode.