is it mk4 astra ? if so the key needs re-initialising ! put the key in the ignition & turn until the dash lights come on . Press & hold the unlock button & the car will lock then unlock . take key out & job done . If this does not work then you may have broken the contacts inside the key when the battery was changed . they are quite delicate & sometimes need packing with a piece of thin card etc . hope this helps !
