I imagine you have squareish fob with 2 buttons? If so that is Lucas alarm system. Each time you disconnect car battery you need to re-syncronise fobs back with the car.
Press 4-5 times unlock button while you are within the car and the fob should resync with the car and you should be able to start the car.
Press 4-5 times unlock button while you are within the car and the fob should resync with the car and you should be able to start the car.