The radio is designed to lock up if the wrong code is repeatedly put in. You'll either be shown LOCK10 or LOCK13 message if it's locked. If it's on LOCK10, press and hold the number 6 button while turning on the radio to change from LOCK10 back to CODE. You then have 3 further attempts at the code before LOCK13 appears. If LOCK13 appears then you will need to take the radio to a Ford Dealer or radio specialist to be unlocked.