The stereo is not working (IN A FORD FIESTA 97) when I put the key code?

I buyed a card Ford fiesta 1997, it coming with a code, the old owner gave me this code but it is not working, also doesn't apear the numbers when I press the bottoms on the stereo the screen just flashing (I think they disconnect the battery from the car) any one can help me with this please, I really apresiate the solution.

Gracias