If the clutch was ok before the car sat, then most likely it's rust on the flywheel - you can get rid of it by simply driving it for a while or if you want quicker results, you can slip the clutch a few times while you're in high gear on the road - but caution, you can glaze the clutch disc from too much heat that way if you go over board. If that doesn't fix it, you could also have a bad pilot bearing or a bad input bearing - this would usually cause the clutch to chatter as well.