That's a shame - does it help to know something like this happens to all of us at one time or another.

My first guess would be a cutworm, although I suppose a slug or snail could have weakened it feeding on the base. No bait or fix is going to be 100% effective, I think very helpful would be the best we could hope for. And Sluggo isn't going to take care of a cut worm - my understanding is that the product Sluggo Plus will.

The first time I ever saw a rabbit in my garden was the first year I grew sunflowers. I came to learn rabbits are very fond of sunflower stems, which isn't helpful if the flowers were the goal ;)

Rabbits and hares are very fond of chewing stems so that the stems fall down and no longer block their view.

Regarding the above sunflower, I would be more inclined to think that it has met its end at the teeth of a cutworm rather than at the jaws of a slug. A very lazy cutworm.

Instead of using snailbait, I'd be inclined to use a physical barrier around future seedlings. The cutworms will only chew through them while they are tenderish, though I don't know what the cut-off point is.