The fibers from the clutch can't get into the transaxle. It could be that when the friction disc gave out it just happened to be the final straw for the synchros.

On second thought I wonder if you didn't reinstall something improperly and now the clutch isn't completely disengaging. Or the clutch could be faulty and not disengaging properly. Possibly you damaged the slave cylinder when reinstalling that?

I'd double check that the hydraulic system is operating properly and that your fork is holding the throwout bearing properly and nothing got bent.