Changing battery 2001 Lumina and radio lock up.

someone (hopefully you) set the 3 or 4 digit security code on the radio. when power is disconnected, like if someone steals your radio, the radio locks and cannot be used. the code can be from 000 to 1999. the hour and minute buttons are used to enter the numbers. 2000 choices - good luck
to unlock the system:
1 turn on ignition - LOC will appear
2 press MN and '000' will appear
3 press MN again until you see the last 2 digits of the code you want to try
4 press HR until you see the first 1 or 2 digit of the code you want to try
5 press AM FM
6 SEC will appear if you got it right - LOC will show if you got it wrong
good luck