vauxhall cavalier 18.i 1995 model won't start orange light flashing?

my car won't start because engine management light on car in garage just now the guy said it is the immobiliser is on but when i got the car i did not get immobiliser with it that was 4 months ago and the car was fine up until now he said i need new key for it and new immobiliser to and it need to go to a vauhall dealer to get it coded in to the car is this true . Thanks to anyone that answers