CHECK THE FLOOR
I know, I know.
It may sound silly, but believe me.
If you are missing for example a piece of the edge or the connection between two different areas, do not panic. It happens all the times. That piece is still around, probably on the floor under you. It's not there? No worries.
In alternative I would suggest looking in one of your containers. If you did the differentiated collection (and you did it quickly because it's very boring) there is a possibility that you put one or two pieces in the wrong place. Well, that's what I usually do...
It's highly unlikely that the puzzle was produced without a piece or two, that would be the last scenario I would consider.