Depending on how badly stuck it is, it can take a lot to break it free.
If you look through the inspection hole, and push down on the pedal, you will likely see the clutch plate expand in thickness. Further travel will possibly show it going with the pressure plate or staying with the flywheel. If you cannot get a thin knife or the like in there to pry it free (turning it a bit at a time) you are left with only 2 options.
1. strip it out. don't do this yet.
2. Do what was suggested by someone previously.
Get her in a paddock with plenty of room. Get her started in gear. and drive her around with the clutch pedal in. You might have to be pretty harsh on her, or she might break free quite easily. Just keep in mind all the time that she will not stop unless you turn off the engine.
The problem occurs because she was parked up with damp clutch linings, and has rusted. A bit of careful slippage will help to clean her up, and get rid of any moisture, so it doesn't happen again. (once she has broken free)