My peugeot 806 car stereo is locked after battery replacement

Sounds like the fuel pump is bad, or the battery needs to be replaced or the alternator. Go to Autozone, they can do a free diagnostic on battery and alternator

Try another battery as the battery may not be charged enough to start the car.

hi
if the car stereo is factory, just contact with the dealership and they will give you the code free

Nope. The dealer is correct.

Your radio is made by CLARION and the only CLARION code available for free (using a serial number ) from anywhere on the Internet is if your radio also  has a BARCODE SERIAL NUMBER beginning with
C7000000 + + + +  on a label somewhere on the metal casing.

 
If not then it will only be available from  your Main Dealer or a Decoding Service or your local Car Radio / Audio Store who will connect a laptop to read/recode the memory chip (EEPROM)

call your local dealership and get the serial code off the radio and have them give it to you

If the stereo is original the unlock code for the stereo is the last numbers of the chassis number

This is a different type coded stereo. These stereos communicate with the car's computer. The dealership is the only one that can recode this stereo to communicate with the car's computer to work. I have seen the dealership charge over $100 for this ten minute process, I have also seen them do it for free. If you are not happy with the recode quote, call another dealership.

make apropriate selections at link below
http://ice.dndservices.co.uk/shop_decode_index.php/MID/42