It's very easy to inadvertently delete a bookmark.  When new data is typed in a cell whose contents are bookmarked, the bookmark may be deleted.  When typing new data in a cell whose contents are bookmarked, make sure the new data is typed within the bookmark brackets.  Bookmark brackets are only displayed when Word is configured to show bookmarks.  When the "show bookmarks" option is selected in Word, bookmarks appear with brackets around the contents.  For example, if the number 123 was bookmarked, it would appear as [123] when bookmarks are displayed.  Of course, it may be easier to just recreate the bookmark using the Formula Builder Wizard.