My Ford Mondeo car 2004 is flashing Security Locked in the Radio screen(600?

My Ford Mondeo car 2004 is flashing Security Locked in the Radio screen(6000cd) after i enterd wrong code ( s/n is m067899 and right code 4488 ) but now i can't inter it.How can i solve this problem..i have tried using inter the right cod mention above , i pushed and hold buttons 6 and 1 or 6 and 2 to now avail..please help