I've never had a problem with my car starting, but today after I'd been driving for a few miles, I came to a stop at a crossroads, and then the engine completely cut out, and I had to be pushed to the side of the road.

When I turned the key to try and start it, there was nothing (silence). The lights still worked, and the battery had a charge. I checked for any loose wires, but couldn't see any.
The car had been fitted with an immobiliser (autowatch) before I bought it, so am I right in thinking it must be the immobiliser, or is there anything else it could be?
Thanks.