If the blade does not move, it's likely the windage screw shaft is broken. You'll have to purchase a new screw and nut, as the nut is peened on the end of the screw shaft. The screw and nut are probably available at Brownells or Numrich....or you could call S&W, 1-800-331-0852.
If the blade is all the way to the left, you will have to unscrew the nut from that end. You can make/modify a small screwdriver to fit in the two slots of the nut and then back out (screw out) the nut and shaft. Use caution, as there is a very small spring and plunger in the screw head that needs to be captured when the screw head is removed from the sight assy. I usually put the sight assembly in a plastic bag when attempting to remove the screw so the plunger can't fly to parts unknown.
Check the FAQ thread at the top of this forum ("Smithing") for more info on the procedure used to change the rear sight blade. You'll have to scroll down some to find the info on the blade, etc.