2000 Zafira rear door not locking / unlocking?

On an extremely cold day the left rear door did not unlock with the central system. Instead of waiting for the car too warm up, my 10 year old 'solved' the problem with brute force.

Now we can lock and unlock this door manually but it does not work with the central system.

What did my son most likely break? How easy / difficult will it be too fix this?

I'd appreciate any tips.