First check if they aren't on the bookmark's trash can. If not then you can try renaming ' Bookmarks' to ' Bookmarksold' and ' Bookmarks.bak' to 'Bookmarks' to see if you can recover them.
There are no bookmarks in the trash, because I accidentally removed them from the trash. I did as you wrote and the bookmarks have been restored. I'm happy that Opera creates a copy of the bookmarks. This is very good.