Yours is a common experience. It happens to everybody. It is a learning experience in itself. It should teach you to keep notes of such things on a separate sheet of paper for future reference. We all learn a lot from painful experience. There's nothing more frustrating than looking through a book and trying to find a quotation you've used or want to us. You have a general sense of approximately where you read it and even whether it was on a left page or a right page, near the top or near the bottom, but once it gets away it's maddeningly elusive. You have to grab it and nail it down when you've got it. In fact, you ought to form the habit of writing down everything you think you might want to use. This applies to your own ideas and your own sentences or half-formed sentences. Memory is elusive. Sir Francis Bacon says in one of his wise essays that the best thoughts are those that come to us unsought. (I can't find the exact quote!!!) But these thoughts have to be written down immediately because they are the most elusive.
Your best bet for your present problem is just to show the author and title. There are many quotes in essays that do not include page numbers.
If you know the edition of the book, you can try searching for it under "Google Books" an this sometimes allows you to preview the book's individual pages. Two drawbacks to this method: you have to know the general area of the book you found the quote in to find it again, and most previews have significant numbers of pages missing.
You could try Googling the pertinent phrase or quote and see if the page number of a Google Books entry comes up that way, but, depending on the book, there may be more than one edition available and page numbers would not be consistent from one edition to another.
The important thing to remember is to cite all your sources and give credit where it is due! So at least you're making this effort. Explain to your instructor what happened and maybe they can suggest how they want you to rectify the situation.