I did have this problem once before after replacing the battery,
and took it to a Garage and they did it in no time although the
Garage that fitted the battery were unable to do it and they sent
me to this other Garage as part of their service.
Ignition on, radio off. Hold AS, turn radio on wait until beep before
letting go of AS. If no beep or dipslay does not turn to 10 ----, then the
set is not getting its ignition live (will still turn on without this but
not let you put code in) You will need to sort that before proceeding.