Try this to get bookmarks back on Windows: C:\Users\your computer name\AppData\Roaming\Opera Software\Opera Stable\bookmarks. This will tell you if your bookmarks are gone. If you have a backup of your C Drive somewhere, then you can go to the same file in it and retrieve your bookmarks. Just copy and paste.
This worked great for me today. Hope this helps. Of course Opera can still loose mine in the future. Unfortunately I couldn't find my speed dial.