stereo and factory sat nav not working after battery swap

hi guys, any help much appreciated.
had to fit a new battery y/day after the old one was totally flat, wouldn't even jump lead start.
after fitting battery, the stereo and the sat nav will not start up, stereo doesn't even say SAFE.
have checked both fuses and they're fine so now i'm stuck.
have checked all other electrical functions and they're all working fine.
its the Audi Concert stereo and its the direction / voice sat nav with the controls on the centre console and the display between the clocks. panel works when i run test sequence but is blank otherwise, i.e. shows no radio / CD functions.
can you advise / help me please?
car is a 2004 A3 3dr 2.0FSi SE auto

cheers for that mate, i've ordered some keys to remove the head unit so i can check the in-line fuse. hopefully, it will be that and a new fuse will sort it. if not i'll try your tip.

clashcityrocker said:

cheers for that mate, i've ordered some keys to remove the head unit so i can check the in-line fuse. hopefully, it will be that and a new fuse will sort it. if not i'll try your tip.

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new fuse fitted and stereo working fine, however i can't get sat-nav to work but i can live without that, it wasn't a brill system anyway.