The immobilizer system will usually set trouble codes if it's a problem. so it'd be interesting to know whether there are any codes stored. What does work? Does it crank? Do you know if there's spark or fuel pressure? Spark is easy to check and you should hear the fuel pump for a second or so when you switch the egn key to "ON" so listen for that. I don't think that changing the battery will set off the immobilizer for sure but I don't think so. Get more info for us if you still have the problem.