how can i fix my keypad immobiliser?

i have just bought a peugeot 106 automatic, i have driven the car for the past 3 days without having to enter the start code, i just went to start car and needed to put code in, i entered the correct 4 digit code and the car did not start, however it started on my 4th attempt, now i have come home and the car will not start again, can anyone tell me whats wrong and how to fix it without spending an arm and a leg