Whats happened is the the car immobiliser has kicked in, Ive had that very same fualt with my 306 TD. next time before you stick your key in have a look for a flashing red light on the centre console. If its blinking take this action.
1. But your key in and turn to the first click
2. there should be a small button by the ignition swicth (if so press and hold it)
3. as your holding down the button turn your iginition key and start your car.
I myself am trying to find a way to remove the factory fit alarm so i can fit my own.
I hope this works if not you can always Email me at counterstrike70@hotmail.com
And ill see what else i can do for you.
David, January 2005