Your tranny might still be okay. The first thing I would do is purge the system of any air.
The Master Cylinder went out on my car. The pedal fell to the floor and I couldn't disengage the clutch. Turned the key off, put into neutral, restarted and couldn't get it into gear.
First I bought a rebuilt master and attempted to install it and bleed the system with no success. I've read several times to never buy anything aftermarket or rebuilt. Makes sense when you consider that the original lasted as long as it did.
I bought a new GM master and installed. I bled the system with a MityVac brake bleeder and my clutch was restored.
I'm still getting air in the line so I'm gonna have the replace the slave cylinder also.
Good Luck
Sly