Without a photo, it's hard to say. Could be slugs/snails, could be cutworms, could be any of a number of other insect larvae or beetles that munch on veggie seedlings. Most of these hide during the day (protection from predators, hot sun, etc.) and emerge at night so inspecting the plants at twilight or just after sunset will often turn up the culprits. And crows in particular are very fond of new seedlings as well.

You can bait for slugs or snails (recommend 'Sluggo' as it is environmentally safe) or use diatomaceous earth to control various other insect pests but you really need to identify the creature before attempting an effective control.

My vote is rabbits. They love beans as they come up and will eat them down to the nubbins as fast as they come up. You may need to protect them until they are larger and tougher where the rabbits won't find them so tasty anymore.