How to start the car from a faulty immobilizer lock?

We used the our Chrysler Neon and everything was fine. When we parked it up we had problems locking the car with the immobilizer. We finally managed to lock the car with it after several attempts, when we went out to use the car again we got the doors open with the immobilizer but we could not start the car. My partner pressed the little arch button above the lock/unlock keys...Does anyone know what has happened or how to fix this??