Passlock alarm problem

I have a '97 Pontiac Sunfire and recently returned from 3 months away. While away I diconnected my battery. As I have done this before and not experienced any problems, I reconnected the battery yesterday and turned on the car to disable the alarm. Everything was fine. Today, however, I unlocked the passenger door and opened it only to have the alarm go off again. This time the car did not restart. It had power, but would not turn over at all. There's a full tank of gas, but I have tried all sorts of things to disable the alarm. The only thing that works now is to disconnect the battery. Does anybody have any suggestions?

I can't remember on this car. Do you have VAts or Passkey? Do you have a chip in the key?