The catalog says the book I want is in the stacks, but I looked and it's not there. What do I do now?
First, make sure you are looking in the right library. On the catalog record, you will see a Library, and then a location, or call number. You have to go to the library where the book is unless the library is SAL. If it is SAL, there is a link on the catalog record to request the item. After you fill out the request form, you will receive an email when the book is available for you.
Next, make sure you are in the right section of the stacks. If the book is at Green Library, check the Library Stack Guide  for the stack location. Next, be familiar with the Library of Congress Call number system. This page explains how books are ordered on the shelves by call number.  Make sure you first search for the books with all the letters in the first part of the call number. Next, be sure to search for the whole number following the initial letters. You will notice that the books around the one you are searching for are about very similar topics.
If you know you are in the right place and the book isn't on the shelf, ask a reference librarian if they can help you find the book. Especially at the end of each quarter, there is a backlog of books waiting to be re-shelved. The book you want may be in the reshelving area. A librarian may be able to help direct you to the reshelving area, where you can search for your book.
If you have gone through all these steps and still can't find the book, fill out a search request form (it is a paper form, not online), and library staff will notify you if they find it.