1. It’s where you think you left it: Look more thoroughly. Repeat three times. Have someone else look, too. (One time I found a friend’s $5,000 check tucked under the flap of a bill.)
2. It fell. Look anywhere below. It’s in the drawer which was open, but then got closed. It hit the floor and rolled. It’s in the trap at the bottom of the drain. (Once I found my missing keys inside the freezer in the ice cube bin.) 
3. Perhaps it hitched a ride. Could it have fallen into a cuff, a shoe, a briefcase? Could it have hooked onto clothes or a shoe? (Could it still be hanging there? Could the contact lens be on your cheek?) Could a pet or child have carried it away? Could another person have moved it, accidentally / on purpose? A spouse might have put it in its “proper” spot. Or thought it was junk and put it in the trash or recycling. 
4. Look up. It didn’t fall, you just put in a higher spot than you recall. Up on a mantel or shelf.
5. It’s right in front of your eyes — if it weren’t covered up. The earring fell into a hamper of clothes, a bucket of peat moss. Or something got moved on top of it (a newspaper, shirt, the mail). Lift everything and look.
6. Could you have done a lame-brain switcheroo? You know, put the glue stick in the silverware drawer, the fork with the stamps and tape? If you were in the kitchen, check the dishwasher and the fridge. 
8. Got a second most likely spot? Repeat steps above.
9. It’s nowhere near the “most likely” spots. Not even close. All of us have faulty memories. It might even be in that one place you’re “sure” it couldn’t be. Retrace your steps. Where else were you? What else did you have done since you last saw it?  Check those places, too. Be sure to check outside. (A friend was sure his cellphone wasn’t on the porch … we found it there after he left.) 
8. Check your favorite places to stash things: jacket, pants pockets, purse, car, favorite drawer.
9. You’re forgetting someplace where you were. Try to remember. If that fails, just start wandering and looking around. Cast your eyes everywhere. Be random. Don’t think. Sure you weren’t in the basement, attic, garage? Sure you didn’t leave at work? Or someone else’s house?
11. Reenact the original situation. It might jog your memory. Or give you new clues. Once my wife put her keys down on her car hood. Then I guess I drove the car. When she couldn’t find the keys near the car or garage, we figured out what must have happened: They fell off while driving. Nope, not in the driveway. Not in front of the house. No way, they could have traveled farther, right? Let’s find out. I did what seemed ridiculous: I drove put my keys on the hood and retraced the route. The keys amazingly didn’t fall off till about a block away. No sign of the keys there. So, why not? I knocked on doors, and, yup, someone had found the keys! 
12. Give up. Relax. For now. Go about your other plans, your normal routine. You might naturally redo what you were doing. Or revisit the place you “lost” the object. Or someone will stumble on it by accident. Have faith. Better yet, tempt the irony of fate by taking steps to replace the item. Or sleep on it. Maybe inspiration will come in a dream. Still no dice? OK, don’t give up. Repeat all the steps above. Good luck!