How do I unblock my in-car (Ford Mondeo Zetec) audio system?

I have a Ford Mondeo Zetec and the audio system is labled as 6000 CD RDS E.O.N. I was asked for its code to turn it on and I did not have it and somebody advised me to enter the last 4 digits of the chassis/number and it did not work. Since I entered it several times, I ended up with a blocked audio system. The message that i get now, each time i try to switch it on is 'BLOCKED'...what can I do to unblock it.