I doubt that it is broken. It's been awhile since I've had the spring out on mine, but I'm pretty sure that it just goes in BESIDE the powder plate. Look at the notches in the side of the plate and line up the spring such that the little bent part of the spring will go into those notches when you slide the plate back and forth. The spring simply holds the plate in either the Open or Closed position.
The end of the spring that goes into the bar housing has a loop on the end. This loop fits around a screw that connects the shutoff slide to the upper cast housing. So put the spring into position with the loop around the screw and then, while applying sideways pressure to the spring to get it out of the way, replace the powder slide. There are two notches in the side of the slide that the spring locks into to hold the slide either in the open or closed position. If you have difficulty, it is easier to do this with the upper unit removed and upside down. There are two screws holding the powder shutoff mechanism to the upper unit. It may help to loosen these screws some to replace the spring. If you turn it upside down, watch that you don't spill the shot and powder if the caps come off.
33t has the right information. The screw holds the bottom plate to the alumunum housing. The powder shut-off plate is a loose part that can come out when the bar is removed. The shot shut-off plate is attached to the bottom plate.
If the powder shut-off spring is separated from the assembly it is broken.