What causes a manual transmission on a 1991 Chevy Cavalier to lock in either 3rd or reverse?

Same thing happened to my friend - there is an "H" shaped (sort of) bracked just below the boot for the shifter - the bolt on the right hand side had dropped off and the left one was loose enough for the plate to move (he was stuck in reverse)... new nut & bolt and a good tighten and he was able to get out of the mall parking lot... .