If a book is not where the catalog says it should be, there are a few possible reasons. It may be awaiting re-shelving, or someone may be reading it right now. The book might also be lost.
You should check a few adjacent shelves to make sure it wasn't put back improperly--sometimes patrons pull out a book, decide they don't want it, and then put it back in the wrong spot. The next step would be to go to the Circulation Desk and ask a staff member to look the book up for you. If that doesn't work, complete a Missing Item Search Request form (this can be done by you through the library's website). Then you will be notified by email when the book is found.