Is this due to the same problem you have already asked about - the security light?
Ok, what we are going to try is a re-learn procedure, but you will need your original master key for this, it cant be a duplicate.
Important: The vehicle learns the key transponder information and/or passwords on the ignition switch transition from OFF to RUN. You must turn the ignition OFF before attempting to start the vehicle.
If, after having done this procedure the car still wont start, then a gm factory diagnostic scanner is absolutely required in order to help you as there is a problem within the security/antitheft system that is preventing key recognition. Not even an independent shop has the right type of scanner to help. Only a dealership can do this, as much as it pains me to say, as I am an independent and am not allowed to purchase one of their scanners.
As you found out, disconnecting the battery doesnt "reset" the computer or erase any fault codes. while that would work on 1995 and older gm vehicles, it wont work on the ones built since 1996. Only a diagnostic scanner can do this.