I would agree with the comments made by Billy above but you might try to free the clutch by this somewhat brutal but hopefully succesful manner:
Push the front wheel of the bike up against a wall. Start the engine.
Push the forks down as far as possible and at the same time pull in the clutch and kick the gearbox into first.
The chances are that the engine will stall or alternatively you might have knocked the wall down but you might be lucky first time.
If it stalls try again as the shock will have jarred the plates slightly.
If you can get the bike into gear with the engine running just hang on and let the plates spin disengaged for a couple of minutes then stop the engine and try again. Hopefully engaging gear will be a bit easier but not necessarily perfect.
Riding the bike subsequently will hopefully clear the plates but if this fails then I am afraid that a clutch strip is the only option.
The 250 MZ clutch is a magnificent piece of engineering which is almost bullet proof so don’t be concerned that you will damage it whilst trying to free it off.