even if the key wasn't recognised by the ecu the rest of the car's electrics would be working fine, including the starter motor when you turn the key. however if there was a problem with the key then the key code light would remain on whenever the ignition was switched on. buying a brava with one blue key is asking for trouble, but i dont think thats your problem. i cant figure out what the problem is without seeing the car, and i wouldnt want to guess. it could be many things. first thing to try is disconnecting the battery for an hour. that will reset the ecu and may get your car started. failing that you need to check the basics, fuel fire and air. i doubt air would suddenly have a problem getting into the cylinders (unless a plastic bag has magically wrapped itself around the air intake, ive seen it before), but petrol and sparks should be checked before anything else. if you try to start the car for a while you can then take a spark plug out and see if it has unburnt fuel on it (smells of petrol, looks damp) if thats the case you probably dont have a fuel injection system problem, at least not one bad enough to prevent starting. with a sparkplug removed try starting the car while watching for sparks. if you get spark then that isnt your problem either. if you get sparks and fuel you can also be sure that the key isnt the problem as the immobiliser would prevent either sparks or fuel (depending on immob, im not sure how the fiat one works, could be sparks or fuel or both). if you're comfortable doing some checks on the car and trying to find the problem that would be a good basic place to start. personally i think its more likely to be electrical as the car ran fine until you switched it off and now it wont start. that almost always means electical problem. finding electrical problems like that can be tricky, where do you start? taking the car to a garage would be the quickest option to getting the car back on the road.