Take a look at the text immediately below 'First Steps' here:
https://www.manualslib.com/manual/778841/Citroen-Berlingo.html?page=174
It shows that radio's the power button has different functions, depending upon whether the engine is running or not and whether a short or long press is used.
With the engine not running, a long press toggles the Mute mode on and off. With the engine running, a short press is used to the same effect, while a long press reinitialises the system.
So, if I was you, I'd be trying different types of pressing on that button.