Lumina-99 Radio Code Urgent Help Pls

A car mechanic removed Battery terminal and now radio is dead. I need urgent help how to unlock I am second owner and first owner did not provide me code. Also pls guide step by step how to unlock?
 
Thanks,

Go to the dealer. Needs to be reprogramed. The previous owner never did have a code.
 
Jen

Thanks Jen, can you please advise me how to enter code I found serial number and some 3 & 4 digit numbers on sticker at steering column.
Any help will be appreciated because I dont have owner manual.
 
Ali

Unfortunately this is something that the gm dealer has to do.