Missing a corner piece doesn’t make the puzzle worthless. Build around it.

An incomplete situation still has most of itself intact. Something important is missing, a person, a resource, an opportunity that did not come through, and the pull is to treat the whole picture as ruined because of the hole. Almost everything else remains there and workable. Stopping because one piece is absent wastes all the pieces you do have.

Name exactly what is missing so it stops being a vague sense that things are wrong. Then ask what can still be built without it. You can keep looking for the piece while you work on the rest.