My cat hid her kittens, and doesn't seem like shes looking for them, or even feeding them??? what do I do?

My cat which is only a year old, just had 5 kittens 2 weeks ago! She took incredible care of them, and was super protective over them. We had a box set up for her and she slept with them all day every day. Today when we came home, we noticed all the kittens are missing! I have been watching her, and she doesn't seem to miss them or even look for them! I cant hear them crying, so I am wondering if she killed them? She was such a good mom, so I don't understand why she isn't going to find them to feed them! PLEASE HELP ME