Stuck pistons are very common on bikes, especially after long storage. When I worked in a motorcycle shop we'd see at least a dozen every spring. If you don't store them with a bit of oil spread around the cylinders when you park it, the rings will rust to the cylinder walls. I see new pistons and a rebore in the future, although it's possible that the damage isn't too bad. Squirt Kroil penetrating oil in the sparkplug holes and let it sit overnight before trying to turn it again. If Kroil won't loosen it, it won't loosen.
