Chech your Audio user guide, it should tell you in there. Failing that, contact a local Audi showroom, and they will be able to help you. You will need to give them the make/model and serial number of your system and hopefully they will give you the four figure code to unlock it.
The code is needed to activate a chip inside the radio. Computer guys work their way round circuits spraying freeze ice onto chips (they go to sleep until they thaw out) until they hit one that makes no difference: this is the duff chip.
Memory chips can be sent to sleep the same way, you just gotta hope when it wakes up it has reset itself to zero or some basic setting, otherwise you are still in the same boat.
Staying overnight at someone else's place and deciding to stache your encrypted radio in their freezer, well, Captain Tw8tty strikes again!