I've had a look at my manual for you.
You need to reset by holding the button for two seconds, as described above by azc
Here are the images for you:
- Sorry for the glare on the important section; Glossy print manual doesn't like flash.
The section in question that is hard to make out in essence states:
If power is completely lost (I.e. Battery removed) the head unit must re establish connectivity with the ECU.
To do so, Hold the power button down for more than 2 seconds.
It is reactivated when a connection is established.
If, after holding the power for 2 seconds to reset etc, It must be taken to a dealer and checked.
Looks like you are up the creek