radio not working after battery disconnected

[rodderspug]

I'm new to CC's. The battery was disconnected, and now the radio wont come on at all, wont even ask for a code. The 'on' button is a small one, separate from the adjacent up/down volume switches, ie, different from the combined large knob in the car manual. The central display/clock is working. I imagine there's a simple solution to this for those with experience of the car, so any advice much appreciated. thanks