Need to get I.D. code out of the car, then go to a dealership and have them call it in to find the reset code......then the reset code is put in, to unlock the radio(make sure you write this code down, in case you lose battery power to the radio again).....
If you know a tech at a GM shop, he can do this for you...otherwise they generally charge you 0.5 hr of their labor rate to do this....
At our shop, if you were a steady customer, they would do it gratis......