vauxhall corsa cdr 2005 not working?

The battery went flat and my car had to be jump started. After that, my radio stopped working. The infopanel is fine and shows up saying the date, time and temperature. The fuse is fine and I've taken the radio out and everything is plugged in fine. I've also tried holding down th AS button method to input the code and nothing happened. It doesn't show up with 'SAFE' or 'CODE' at all. Anyone know what's happened and what I can do?