Electrical mystery

[SideShow Bob]

Along the line of jslo's thought, does the switch have a "On and Off" embossed into the lever ? Or, is it blank when in either position ?
Also did you throw any switches at the opposite end of the darkened section ?

If it is a miss-wired 3-way switch, the problem switch and it's mate will have to be in a certain position for the lights to be on.
Put the switch back into the up (what you consider on) and try flipping the switches at the other end of the area, one at a time to see if you can get the lights back on.