Modern cars are funny. In an effort to reduce the incidence of car radio thefts, the devices need a code entered in them every time you remove the live power feed. In practical terms, if you disconnect the battery, or it runs flat, you have to re-enter the code to get some music.
That's not a huge problem if the car is new. On a second hand car it's no so good.
I disconnected the battery when I fixed the mileometer on my Peugeot 206, the radio stopped working. Luckily for me a previous owner had taken the time to write the four digit code in the manual. Unfortunately it was the wrong code.
I know this because when I entered the digits the radio didn't unlock. And when I did it again, twice, it locked up properly. I was not a happy bunny. Searching the web didn't help much either - no one seemed sure what to do. So I read the manual instead. Properly. Eventually. OK after 2 hours fruitless Googling...
First the right code was required. If you don't have it, the trick is to remove the radio, look at the numbers stamped on the case and then enter them into one of the many sites that will be presented if you enter "unlock peugeot radio code" into your favourite search engine.
To remove the radio, poke a long thin thing into the holes either side in the faceplate. A screwdriver worked for me but a knitting needle might be just as good. This releases the catches and the entire unit can be carefully slid out. Do not pull hard or quickly as it's still attached to the car with a lot of wires. You don't need to unplug anything as the codes are on the top of the case.
To return the radio, compress the long thing catches either side of the case, the ones you opened with your screwdriver, and slide the unit back. It should lock in position.
Once you have the correct unlocking code, power up the car (position A on the ignition) and enter it. Then listen to those bangin' tunez.
Unless that is, the display still shows "Radio locked" as soon as you turn the key. That means you've entered the wrong code too many times. The solution is to use the spare key to turn the ignition to A (one click) and leave it in this state for a whole hour. Since you still have the main key handy, lock the car up and go and have a cup of tea. If you only have one key, sit in the car and read a book for an hour.
At the end of this period the central display should be asking you for the code. Enter the right code and all will be well. Then write the code down somewhere you'll remember it if required again.
Oh, and if you've read anything online about sticking the radio in the freezer to achieve the same effect, forget it. That hasn't worked for many years. In fact you'll probably stuff the thing up as moisture will form inside and do the electrics no good at all.
Legal note: This is an accurate description of what I did. If you chose to follow these instructions and things don't work, it's not my fault. Sorry.