Go to your Opera profile folder (you can see where that is by looking at "About Opera" in the menu.
In that folder should be a file called "Bookmarks" (no extension) and a file called "Bookmarks.bak".
If "Bookmarks.bak" is a lot bigger than "Bookmarks", it probably still contains your lost bookmarks.
Copy "Bookmarks.bak" to the desktop, rename it "Bookmarks" (lose the extension) and copy it back to the profile folder, over-writing the existing file.
Good luck!
I've no idea why sync would lose your bookmarks BTW, but keep the version of the "Bookmarks" file that you have on the desktop somewhere as a backup in case it happens again!
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