First you will have to disconnect your iPhone from the USB cable, do not unplug the other end of the cable let it remain connected with the computer.
Turn of your phone; this can be done by holding the Sleep/Wake button until the red slider emerges.
While turning off your cell you will have to reconnect the USB cable with your iPhone. Your phone should turn on while being charged. Charge it for at least 10 minutes and then hold the home button to turn your iPhone on. As soon as the screen appears release the home button.
In order to view the recovery message open your iTunes and you will see a dialog box showing the message ‘iTunes has detected a iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes’
If the above stated message does not appear then repeat the steps mentioned earlier or contact Apple Support online.