Check the detent spring that holds the shifter in place, it probably broke. Of course that will require a rebuild of the transmission. Not many people are doing that nowadays. I found a shop whose owner is in his late 70's and only does older stick transmissions (that's just what he likes to do). I gave him the trans last Nov and got it back two weeks ago. He just keeps getting transmissions to be rebuilt daily.
The shop owner allowed me the courtesy of watching the disassembly and the reassembly.