Electrical wiring can be very confusing, even to some electricians. Electricity is inherently dangerous; the more you understand the better prepared you can be. Open neutrals -- outlets with a disconnected neutral wire -- can be deadly if not understood.
If you notice that you have an outlet or perhaps a light switch that is not working, but other devices in the same room are, you may be dealing with an open neutral. In order to verify this you need to remove the plate that covers the wiring for that outlet. Take all pertinent electrical precautions when doing so and verify if the neutral (white) wire is connected to the receptacle; if it isn't, then you have an open neutral.
The neutral wire could potentially disconnect from the receptacle for to a few reasons. One reason this could happen is disturbance from vibrations; every vibration from doors shutting or blowing wind can gradually lead to the wire coming loose. Another common cause is rodents getting into the walls and chewing through the wires.
The purpose of the neutral wire is to return the current from the device back to the source of the electricity. If the neutral wire is not connected, or "open," it will allow a potentially large charge to be stored in the device it's connected to. If this is the case and you touch that device you could be electrocuted.