Hi there! I'm a newbie to gardening so please bear with me. I'm growing a fern in a pot indoors (can't remember which kind it is exactly). Two little fuzzy sprouts popped out and they grew to be an inch to two inches long and then... they just disappeared, as if cut at the base. You can see where they were growing but they're just missing. It happened overnight, it was sitting on our counter and our dog was Locked in our room overnight so I know she didn't eat it. Could it have been aphids? Can they do that? There are little bugs in the dirt that I can see moving around. Here is a picture to see what I'm talking about.

Possibly a mouse or a pet? It looks like it was bit off. Aphids are sucking insects, they don't eat the plant by biting down. There is a good chance more fronds will grow in.

could also slug, they can hide under the pots or within soil, and mostly nocturnal

Since you are indoors the slug might be under the pot or just inside the drain holes.