Sounds like you need to input the radio code . i would look in the glove box for the code either on a sticker or a card or ask them if they know about the code .

Instructions
Deactivate Theft-Deterrent Feature

• 1
  Place the key in the ignition and switch it to the "RUN" setting.
  
  
• 2
  Press the volume control knob in to turn the stereo off, if it is not already off.
  
  
• 3
  Press the number "5" and "6" button on the radio simultaneously for approximately five seconds until four dashes ("- - - -") appear on the display screen. You must then enter the four-digit unlock code that you originally assigned to your vehicle. If you are unsure of the unlock code, contact your local GM retailer.
  
  
• 4
  Press "SEEK/SCAN" upward to set the first number.
  
  
• 5
  Press "SEEK/SCAN" downward to set the second number.
  
  
• 6
  Press "TUNE" upward to set the third number.
  
  
• 7
  Press "TUNE" downward to set the fourth number.
  
  
• 8
  Verify that the code on the display screen matches your stereo unlock code, then press the AM FM button. You will see "REP" will display on the screen. Enter your code a second time for confirmation. "OFF" will appear on the screen, confirming you successfully deactivated the theft-deterrent feature.