I didn't see this. My guess is that the pins on the shifting forks (one of them) popped out of the shifting drum. Or, the drum itself is no longer ratcheting properly (more likely).
Chances are that you can in fact repair/restore this transmission. Maybe there is a burr on the ratcheting mechanism, and all that is needed is some careful filing and reassembly. This is my best guess.
If this is true, you SHOULD be able to break it loose without disassembly!!!
I would highly recommend attempting to free up the ratcheting shifter drum BEFORE disassembling the two crankcase halves (only way to take apart the transmission).
It may be possible to work it back and it may not get stuck again. Try to figure out what is causing the ratcheting mechanism from getting engaged by the linkage between the shifting shaft (gear shift mounts to this) and the end of the shifting drum. I'll bet this is where the issue is.