how can i fix my zantia 2.0 HDI SX reg. 24/1/2001 ?

When I turn the key the immobilser light comes on and the audible warning sign indicates there is a problem - any idea how this can be cured? Someone suggested using the spare key but when I do this it is exactly the same reaction on the electronics. Obviously the car is not worth a lot of money so not worth spending a lot on it although it has been brilliant until now.