Hello, my name is Mark and I will try help you.
If the immobiliser light is staying on then either the key has a faulty chip in it, the receiver is faulty, the ecu is faulty or the wiring for the system has a fault.
If this has only happened recently then a possible cause is that you may have changed the battery in the remote and knocked the chip out of place.
The first thing to try if possible is the spare key. If this too doesn't work then the problem is with the car. If it does work then the problem is with the key.
If you have no spare key and the chip has not been dislodged then your only option is the use of a scan tool with the same functions as the main dealer tool.
You may require a new key and then have it enabled so the car recognises it.