Why would a single electric receptacle stop working?
Some possibilities:


it's controlled by a wall switch and you turned off the switch.
the receptacle is old and worn and no longer making a good connection to the plug.  The main symptom of this would be the plug being loose in the socket; it's unlikely to happen suddenly so you might have noticed it beforehand.
the receptacle was overloaded and got damaged.  You might see some visible signs of charring or smoking around the receptacle -- especially if you remove the cover plate and look inside the electrical box -- if this is the case.
a loose connection in the wiring has finally failed (this might be in a different part of the circuit).
it's on a completely different circuit from the other receptacles in the room.  If you're adding an outlet, the most convenient place to get power from may not be other points in the same room; it could be from an adjacent room's circuit, or even from a different floor in the house.


A non-contact voltage tester will help you find out if you're still getting power to that receptacle.  If so, replacing the receptacle will probably fix the problem.