Our Blaupunkt Boston C31 stopped working suddenly. It is silent, and most of the buttons have stopped working. If I press and hold the menu button, I get the time displayed as expected. But just pressing the menu button for 'menu' has no effect. The mute button causes "mute" to appear on the display. But pressing mute again does not un-mute it. Obviously, it has not lost all power. We have the manual for it. Any ideas?