That's not nearly enough information for anyone to be able to help you.
Does your document jump from p. 118 to 120? Is the right master page applied to p. 119? Did you accidentally override the frame with the page number in it and then accidentally delete that frame? Doesn't reapplying the master page work, or (in case it does nothing) first applying another master page and then your original one? Is there an object on top of the page number? (Possibly a large white box?)
You see, tons of questions but all you tell us is 'the page number is missing'.
Are you using InDesign CS5.5 with book (INDB) files?