With a door that is not used very often, the door must be not be lying on the ground to show that it is a door and not a new building.

Chapter 313 Section 4

In the olden times, a door was made of a many pieces of wood. Therefore one board did not necessarily look like a door but rather a wall for a new building and if detached it is prohibited to be used on Shabbos.

Chapter 313 Section 5

With certain hinges on utensils, once the hinge falls out it is questionable if it can be put it back in as it looks like you are putting the final touches to the utensil and are therefore ‘maakeh bipatish’ [finishing a utensil].