Immediately contact the library unit to which you returned the book.
If you believe that you returned the book, ask the library to "place a search" for it. If the book is found during this search, it will be removed from your account and all corresponding overdue fines -- should any have accrued -- will be waived.
If the book is not found, subsequent searches will be performed over a period of several weeks. If the book is still not located after three searches, it will be considered LOST. The respective library unit will give you your options for replacement.