1994 Toyota Camry Crankshaft Pulley Replacement

I don't know how this happened and I've never seen it before, but the pulley attached to the crankshaft slid out away from the crankshaft. Now I don't know if the entire thing needs to be replaced or not. We pushed it back in with a hammer, but when we try to tighten the nut in front, we can't because the crankshaft turns with it. How can we stop the crankshaft from turning in order to tighten the pulley? Do you think the pulley is broken because it came out like that? We first noticed because the alternator belt kept falling off. We replaced it with new one, but still falls off and then we realized that the pullies were not alligned.