Start the engine and allow it to get up to operating temps. Sometimes the clutch facing will release from the flywheel on its own. If the car is driveable (safely) then driving it in slow-to-fast bursts with the clutch pedal depressed will often times cause it to break free. If you don't feel comfortable starting it in gear and driving it, then you can try placing the rear axle on jack stands and starting the engine. Then allow the rear wheel(s) to turn with the clutch pedal depressed and try jabbing on the brake pedal to free it.
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This advice worked for me. Jacked up rear end, started car in gear, than hit the brakes. Freed right up.
Good luck.