At first thought, sounds like a broken guide rail or shift fork. However, it could be in the cables themselves, especially where they are attached at the bracket on the trans. Check to make sure that the cable ends are still attached to linkage, and the bracket that holds them is secure.

Sometimes the rubber "eyes" that connect to the linkage that comes out of the tranny deteriorate.

If all that looks good, you've broken something internally. If that's the case, yank the trans and pull it apart.

If it's a 1G, it's not all that bad of a job. A 2G is a different story however...