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For either shifter lever to move, at least one must be in its "neutral" position (which is the mid point detent for either lever). The interlock plunger inside the cover prevents one lever from moving unless the other is in neutral.
If you have both levers centered in their neutral positions, and neither one will rotate, either the shafts are frozen/rusted or something has somehow jammed in the detent interlock part inside the cover. I guess this could occur if one or both of the detent balls (3/8" dia) have popped out.