First make sure that the code has not been noted somewhere within the car (underneath the ash tray, glove box , rear of sun visor etc)
 
 
To see if your code is available here  for free then the radio will have to be removed from the dashboard (and if you are lucky, someone may have written the code on the casing ) .
 
If not then POST A NEW  QUESTION
 
quoting the radio MAKE+MODEL plus ALL the LETTERS+NUMBERS on ANY LABELS ,BARCODES or STAMPED IN THE METAL CASING
 
Also quote the MAKE of CAR
 
note: Unfortunately not all radios can be decoded using the Serial Number
 
Otherwise try your Main Dealer ...........or take it into a Decode Service who will plug in a laptop to read/recode the memory chip (EEPROM). This takes a few minutes and shouldn`t be too expensive