Put the bike on the center stand, or if you don't have a center stand, you can do this while riding, but be careful, you don't want to break bones in the process. Get the bike going in second gear, then click it into third (no clutch just shift when the RPMs are right). Then pull the clutch lever and jam on the rear brake. This should theoretically free up the stuck clutch plates. If not, then you gotta open it up, but at least you tried.