How can i get my boot on my 1999 holden astra to unlock ?

Hi i have a 1999 4 door holden astra and i have put the key in numerous times and it will not unlock before you had to put the key in and push the actual lock in but now you push it in and turn it all around and nothing and i cant get into the boot at all we have only had this car for a couple of weeks is there a boot pop latch for there cars and if not how much do you think it would cost to fix please