Bike not shifting

On my 1982 kz440 I pulled off the highway in neutral, went to shift into first and the bike would click into gear but would not move at all. What could be my issue? 

It seems like the clutch is engaging and disengaging fine, and the shifter clicks up and down through the gears, neutral light shuts off, but it will roll in any gear without the clutch engaged. I'm so confused. It sounds like something is broke inside the engine when I roll it too. I will take it apart after work and post some pictures. How hard is it to replace the entire transmission? 

Check the sprocket on the output shaft.

Did the countershaft sprocket come loose?

I have no idea, Im at work til 5 and won't be able to look at anything til after that. It's a belt driven bike as well if that changes anything. 

Lots of possiblities.
Maybe the belt has stripped some teeth .

I would look at your drive system first.  Is the belt tight enough?  Is it in good condition?