The upper plate rotates in a groove in the distributor housing. The plate has three tabs. with the cap and rotor removed, looking straight down at the distributor houseing you will see the groove with three reliefs, indentations that allows the upper plate to be removed. There is a spring between the upper and lower distributor plates, This spring holds tension on the upper plate for stabilazation of the tabs on the upper plate where it rotates in the groove in the housing . If the cap is removed, the upper plate can be rotated until the tabs align with reliefs , this causes the spring loaded upper plate to jump the groove in the housing. The plate may have somehow jammed the plate tabs in the grove so that it no longer rotates. You should be able to carefully push down on the plate and work the tabs back in place so that the upper plate will function again.