Mazda 323 Rear hatch lock broken
 

Hi, My little Mazda 323 rear hatch recently refused to shut and it appears that a small piece of blue plastic with a spring has fallen out of the lock. The lock is stuck in the shut position (why the boot won't shut) and is not responding to the key, handle or remote locking.
I know nothing about cars (if that wasn't already clear :p) - any advice on how to get this fixed or how much it should cost to get it fixed at a mechanics??
Thank you so much for your help!!