renault laguna initialle 2.2 02plate sat nav fault

I bought a renault laguna estate fitted with sat nav.The car had a flat battery before i bought it and it appears that the radio coding was lost when the battery was changed.To enter the code i need to see the sat nav screen but it stays blank(dark).After looking at the handbooks they suggest that (1)The ignition is not on.it is! (2)The fuse may have gone on the radio.I cant find a radio fuse and anyway the radio makes a bleeping noise when some buttons are pushed. Is it possible that there is a reset button somewhere on the sat nav system?Reboot? Any help would be appreciated.