My most frequent knife-eater is the pockets of my pants. I lost my Case Peanut for about a week and it was in the coin pocket of some jeans that I had laid over the back of a chair instead of tossing in the clothes hamper.
Others have ended up (courtesy of pants pockets) going through the laundry. Some clattering around in the dryer, but at least one that ended up in the rubber gasket around the door of our front-loading washer. Which has some sort of secret compartment where stuff ends up sometimes.
I had a couple of knives I thought I had lost in an office move - I searched everywhere in the boxes I moved stuff in. They were cheap knives but I hated losing them anyway. Then about a month later, I was looking for some other tools in an old work bag and found that I had put them in a zipped pocket of that bag so that I wouldn't lose them in the move. And of course, had completely forgot about where I put them.
I gave a knife to a friend who used it quite frequently for hunting, until he left it lying in a corn field in the next state after using it to clean a deer. The lady who owned the land found it two weeks later out in the corn field, and he got it back.
Try looking on any potential horizontal surface where you might have put it temporarily - on top of shelves or furniture, the fridge, inside a cabinet, anywhere you might have laid it down without thinking.
Ask your family members. I had a knife disappear for several weeks until my wife told me she had put it in the kitchen tool drawer (which I never keep knives in).
Beyond that - spend time mourning your loss, realize that it's just some metal and plastic, and learn a lesson about becoming too attached to material things. Then buy another knife.