The radio in my 1998 3.0 Litre Peugeot 406 Coupe had a pre-existing fault in the form of a broken radio on/off switch which meant that the radio would be on all the time that the car ignition was on. This wasn't too much of a problem as if I didn't want to hear the radio then all I had to do was turn the sound down.
Recently the car was in a dealership for some work during which time they disconnected the battery. When I collected the car from them the radio would not come on at all and there was no "beep" noise and the prompt for me to put in the 4 digit security code (which I know), did not come on. On previous occasions when the battery has been disconnected the re-connected, I've always been able to put in the 4 digit code. Can anyone explain what is likely to be wrong and what the solution might be?

Stephen Birney, June 2021