First question has to be, did you (or anyone else) do any work on the Brakes before this happened?
When you say you haven't lost any fluid, do you mean the reservoir level is OK?
As reneduba said, a blocked feed hole or cap vent may be enough to cause this problem.
If it went very suddenly, it could be the Master Cylinder seal, although I think some M/Cs have a back-up seal, and it's also a dual circuit system, so the Brakes really shouldn't fail completely.
Could the pedal have become disconnected, or a part broken?
Does it feel like there's absolutely no resistance to it's movement?