This post applies to pre-2004 models; it's probably not relevant to newer wheels.

A short steel shaft connects the lock mechanism to the bonnet release latch.
Shaft diameter reduces at each end. A circlip is inserted at the latch end to keep correct pressure on the lock end. If this ring is missing or worn too thin the inserted key will not engage.

With luck and perseverence putting upward pressure on the key and trying to pull the lock towards you may bring sudden clicking joy.

If not, underneath the car is a thin access hatch between bumper and radiator.
This is secured by four plastic pop rivets which can be unscrewed.
Very little of the latch mechanism can be seen from underneath but poke a stick or long screwdriver up through the gap you've created by opening the hatch. Again with a little perseverence, by tapping or scraping your implement close to the lock/latch while another person keeps working the key you will hear that happy release sound.

Once the bonnet is open you will see exactly what the problem is. By gripping the shaft with long-nose pliers and putting pressure towards the lock the key will work OK.
If you're familiar with circlips pop a new one on and that's it. If not, wrap any piece of strong wire around the latch end where the ring should be and nip it tight with pliers.

As in the earlier post, do not close the bonnet without using a screwdriver to simulate
closure and ensuring the latch releases freely with several key turns.