I wanted to lock the trunk the other day and I got the key in the lock, but turning around was not possible.

After injecting with WD40 and fiddling with the key, it eventually managed to turn the key around for a quarter. Pulling it out certainly didn't work. 

Then disassemble the lock and see if it's commonplace. I was surprised how much rubbish and dirt can get into such a lock! To remove the rust, I put all the parts in strong vinegar for over a day. Another acid will work, as long as it's an acid. This reacts with the rust and so you can easily remove the rust.

After the acid bath, rinse well under the tap and continue cleaning the lock. With me, the slide was fixed, but with some drenched and back and forth I got the slider common. 

In this slide there is also a cap with a spring behind it. With me, this cap was stuck in the slider. To get it loose, I used a very thin drill to pierce a hole in the cap so I could pull it out. 
Then clean thoroughly and close the hole with the soldering torch and some tin.

The whole lock is then put back together (don't forget the little feather) and it works as it is meant by Citroen.

What I've read on many forums is that you shouldn't put grease in the lock. Only graphite powder and I share that opinion. Probably the previous owner of my 2CV used grease for the lock.