fell back in the panel? your mechanism has fingers that hold the plastic selector arms in place--- sometimes those fingers get spread & the plastic just falls out--. if your saying your mechanism doesn't move --then that is another problems all together.
take the plastic shift selector off-- & look at the fingers a gentle clamping force with pliers should close them up to hold the plastic levers in place.
also spray the mechanism with a lubricant -- i'm sure these go bad-- but usually the mechanism works.
Lawrence