If it was working, then suddenly isn't here's a possibility: if you can remove the plastic lens cover do so VERY carefully and look for a small black "chip" laying in the cover. Its very small, about a quarter inch square and flat. This is a surface mounted bridge rectifier placed on the circuit board and is supposed to fail if temps in the lamp get too high. It just melts the solder that holds it to the board thereby opening the entire light circuit...a purposeful failure. It also allows you to connect either wire without regard for polarity of voltage. Be very careful opening the lens. The chip would be laying inside and would easily fall out and you'll never find it. If you closely examine the circuit board you would also see a spot where it was soldered. Just a guess. Solution: new light unless you're into surface mount repairs.
Cedar Creek 36CKTS
F350 Lariat ("Big Blue")