How can I fix my immobiliser?

I have a Ford Escort Cabriolet with an immobiliser fitted to it & in the past few days I have had trouble starting the car as the immobiliser wont turn off. I know nothing about cars or immobilisers but I do know in order for me to start the car the red light on the dash has to be constant and I do that by connecting the device that came with the car key. Sometimes it works but recently it has taken 10 minutes or more to stop it flashing. I bought the car second hand a couple of years ago with the immobiliser already fitted and I dont have a clue what make it is. Its driving me mad as I now cant rely on the car to take me anywhere and more importantly get me back home. I am a female and for obvious reasons do not want to be stranded anywhere especially at night. Can you help me?

Amanda
August 2009