Audio Systems: locked car stereo, 2004 chevy cavalier, vehicle vin number

Question
QUESTION: I went to work and my radio was working fine, but when i got off of work the display says locked and i can't do any thing but change the time, it's a 2004 chevy cavalier. What can i do i bought it used and it didn't come with a book?

ANSWER: what is used the car or the radio ?

---------- FOLLOW-UP ----------

QUESTION: The car is used the radio came stock.

Answer
for some reason the radio lost the vin number of the vehicle
NOW IF THE RADIO IS OK YOU NEED TO TAKE TO THE DEALER THEY WILL USE THE GM SCAN TOOL and enter the vehicle vin number on the radio
if not the radio will stay lock or work for 2 seconds and show  lock

or before you do that remove the battery overnight and try in the morning

there is no other way to ender the code in this radio
dealer only