I've seen a few things that can go wrong with Fiat door locks.
When you say 'the lock mechanism is disengaged' do you mean you've unlocked the door using the key?
If so, is the interior door lock button up? If this button remains down, the lock is still locked as a latch (part of the lock) is still engaged which will prevent the horseshoe shaped latch from rotating even when the inner or outer door handles are pulled.
If the interior door lock button is up, the lock is unlocked. I've seen 2 things then prevent the horseshoe shaped latch from rotating - either it has seized/frozen in position or the spring that makes it rotate, when either inner or outer door handles are pulled, has broken. You could try using a lever or rubber mallet (soft faced hammer) with either door handle pulled to see if you can make the horseshoe shaped latch start to rotate - you might be able to spray some release type spray onto the lock inside the door to free up things but this horseshoe shaped latch is usually lubed with the door open so a better idea might be to spray some lube down between the door and door pillar.
If you can open the passenger door, you can check out the lock mechanism, see how the different parts operate, how the door handles and lock button are connected/operate and see where lubricant needs to be applied.
Unfortunately, the screws that hold the door lock in place are only accessible with the door open.
AL.