Xsara immobiliser

I've somehow triggered the engine immobiliser on a 99 Xsara and I don't know how to get it going again.
The background is that the car belonged to my father who died
recently. I've used it a couple of times without any problems but, on the most recent occasion, the engine wouldn't start and I noticed an illuminated yellow light with a key symbol on the dashboard. I assume this tells me that the engine is immobilised.
Presumably I did something wrong but how do I fix it? I have three keys for the car, two plain and one with a stud which has worn away. I also have a small card which has an alphanumeric code on it which may well be relevant but I don't know how. Unfortunately, the manual is in his home (I assume) and I won't be able to get there for a couple of days.
I got locked in the car as well but the doors apparently unlocked themselves before I suffered too much embarassment. I assume this is related.
Can anyone advise me?
My email address is geod (at) dial dot pipex dot com
You know what to do.
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