In general, contacts are in their normal condition when ALL energy is released. So, for a limit switch for instance, the contact are in their normal state when the lever is not being pressed (that is, the internal spring is storing as little kinetic energy as possible). When the limit switch is activated, the spring is being compressed (storing more energy) as it tries to return the lever to the relaxed position.
The same holds true for pressure switches: no pressure differential (the pressure inside the switch is the same as atmospheric pressure outside the switch) means that the contacts are in their normal position. Positive pressure or negative pressure (vacuum) causes the internal mechanism (usually a spring) to store energy. When this occurs, the switch is not in it's "normal" state and the contacts change states.
That being said, there ARE exceptions. The most prominent exception is with safety switches (door switches). The NO/NC nomenclature tends to be backward when dealing with these kind of switches. The thought being that since the switch has to be activated in order for the safety circuit to allow a machine to run, it is then in it's "normal" position. Although I understand this, but I don't like it.