1. Could've been stolen
If everyone knows that's here you always put it, and they see you put it there everyday then there's a good chance it was stolen.
2. Look around the house
Maybe you've misplaced it in your bedroom or whatever. Yes you always put it in one place but who knows, you just might find it somewhere else.
3. Double check
Who knows, it was probably in your bookbag all along
4. Someone borrowed it and you forgot.
Another possibility. If you can't find it then check in with your friends and family.