How do I reassemble the key?

First, find the key (point B, up the ladder).

You also need a ladle (in the kitchen, point H).

Now, head to the blacksmith (point C) and get a couple of items there (forge
hammer and hacksaw).

In the adjacent room (point D) there is some charcoal you need.

Now, put that charcoal in the brazier in the blacksmith room.