Corsa stereo wont come out of saying " display safe"?

My son has just replaced his stereo in his corsa , like for like, but after turning the ignition on after 30 seconds the words "display safe" show and nothing will remove them. We have a code to enter but cannot get that far, the stereo and radio both work fine but the display will not show track number playing, volume etc etc, can anyone help.

linda
April 2013