EXACT same thing happened in my R33 GTS-T this arvo. Pushed clutch in to go from 1st to 2nd, heard snapping/crack sound, clutch still engaged and cant change gears.
I've had the exact thing happen with my old VL too, it turned out that there's a spring inside the master cylinder that isn't only for pushing the pedal back out and it had snapped into 3 pieces. It it somehow required to allow the fluid to be pushed towards the slave cylinder too.
I'm hoping that I have the same problem with my R33 now. Either that, or like you said the slave cylinder could be farked somehow, or the pivot joint broken.
If you're up for it, remove the master cylinder and dismantle it. Check out that everything is as it should be inside. From what I remember, they're fairly simple - it should at least be easy to see if that's the problem. Worst case scenario after doing this is you've ruled it out as the problem and given it a clean anyway