Problem:
You can select a floor but it is not actually visible in any views not matter what visibility settings are.
Cause:
The Shaft entity only cuts floors, and if a floor is within a shaft it won't be visible at all.
However all other entity types (walls, doors etc) aren't cut and so are visible within a shaft.
Solution:
Delete or edit the Shaft.
You can't use a Shaft to cut a hole in a floor and then put another floor into the hole.